Outranking Methods as Tools in Strategic Natural Resources Planning

Two outranking methods, ELECTRE III and PROMETHEE II, commonly used as decision-aid in various environmental problems, and their applications to decision support for natural resources management are presented. These methods represent 'the European school' of multi-criteria decision making (MCDM), as opposed to 'the American school', represented by, for instance, the AHP method. On the basis of a case study, outranking methods are compared to so far more usually applied techniques based on the ideas of multi attribute utility theory (MAUT). The outranking methods have been recommended for situations, where there is a fi nite number of discrete alternatives to be chosen among. The number of decision criteria and decision makers may be large. An important advantage of outranking methods, when compared to decision support techniques most often applied in today's natural resources management, is the ability to deal with ordinal and more or less descriptive information on the alternative plans to be evaluated. Furthermore, the uncertainty concerning the values of the criterion variables can be taken into account using fuzzy relations, determined by indifference and preference thresholds. The diffi cult interpretation of the results, on the other hand, is the main drawback of the outranking methods.


Introduction
Nowadays, natural resources are more and more often managed for multiple uses.Decision-makers have not only economic objectives but also those of amenity and non-market values of recrea-tion and nature conservation, for instance.Generally, both economic, ecological and socio-cultural sustainability are required.Multi-functionality calls for multi-objective natural resources management planning and decision support.In addition, there is more and more often a need to take into account aims of multiple decision makers or participants in natural resources decision-making.This, in turn, calls for group decision support and participatory planning.
Research on natural resources planning has answered these challenges by applying and developing decision support methods and techniques for multiple criteria and participatory planning.Attempts have been made to solve planning problems on one hand by quantitatively and on the other hand by qualitatively oriented approaches.Both of them have their benefi ts and disadvantages (e.g.Kangas et al. 2000).In practical planning processes, all the information available should be able to be dealt with -no matter were the information descriptive or numerical.Consequently, participatory and multiple use natural resources planning call for analytical decision support methods by which the various kinds of information can be dealt with in a versatile way.
Recently, methods based on the very ideas of multi-attribute utility theory (MAUT) have been developed for purposes of natural resources planning.Many applications and extensions of the Analytic Hierarchy Process (AHP), developed by T.L. Saaty (1980), are examples of such efforts (see, e.g., Kangas 1999).From the viewpoints of multiple use and participatory planning, the AHP has several advantages.Both objective value information, expert knowledge and subjective preferences can be utilised by using it.Also, qualitative criteria can be applied in the evaluation of alternative plans when the AHP is applied.The AHP is based on a theory of ratio scale estimation (Saaty 1977), and by using it pairwise comparisons of qualitatively expressed measures can be transferred into a ratio scale.Other MAUT and MAUT-related methods usually require values of the criteria to be measured straightly in ratio scale and quantitatively.
Crucial problems with most MAUT and MAUT-related methods include handling of uncertain or fuzzy information, and coping with information expressed in other than ratio or interval scale.In natural resources management, descriptive expressions instead of quantitative measures, and qualitative or ordinal information are frequently faced.Furthermore, in natural resources management, a great share of information gathered or produced during any planning process is typically uncertain.The AHP method, for instance, has been further developed in order to meet the needs of analysing uncertainties (e.g.Alho and Kangas 1997).The same holds with approaches based on mathematical programming (e.g.Mendoza and Sprouse 1989).These lines of development work are still continuing, and many advances are still required for elaborating the methods.
Outranking methods serve as one alternative for approaching complex choice problems with multiple criteria and multiple participants.Outranking indicates the degree of dominance of one alternative over another (e.g.Rogers and Bruen 1998b).The outranking methods enable the utilisation of incomplete value information and, for example, judgments on ordinal measurement scale (e.g.Rogers and Bruen 1998b).They provide the (partial) preference ranking of the alternatives, not a cardinal measure of the preference relations.The outranking methods have been used, for example, for choosing the solid waste management system (Hokkanen and Salminen 1997a,c), for locating the waste treatment facility (Hokkanen and Salminen 1997b), for nuclear waste management (Briggs et al. 1990), for irrigation system evaluation (Raju and Pillai 1999) and other Environmental Impact Analysis (EIA) projects.
In outranking methods, strong assumptions concerning the 'true' preference structure of the decision maker are avoided.It is not necessary to assume that a utility function exists, or that it can be described with a certain functional form.The question is, whether there is enough information to state that one alternative is at least as good as another.In voting theory, the alternative a is deemed better than alternative b if the number of votes (or criteria) indicating that alternative a is better than alternative b is larger than the number of votes indicating the opposite.The voting theory can be seen as an ancestor of the outranking methods (Vincke 1992).
Because the very nature of outranking methods is different to that of mathematical programming and of most MAUT methods, it could be useful to examine them in natural resources management.This article is a report of such an examination, concerning especially the methods PROMETHEE II (Brans et al. 1986) and ELECTRE III (Roy 1991).These two outranking methods were applied in a case study based on a real-life strategic planning process of State owned natural resources in Finland.In the initial planning process, an interactive decision analysis method making use of MAUT was adopted by the Finnish Forest and Park Service, which governs the case study area.The planning case is reanalyzed in this study using the outranking methods.The original application is also briefl y presented below for the sake of comparison and as background information on the case examined.

The Pseudo-Criteria
In PROMETHEE II and ELECTRE III outranking methods, the criteria are treated as socalled pseudo-criteria (Brans et al. 1986, see e.g.Hokkanen and Salminen 1997).This means that a threshold model is applied to the original criteria value.If the criteria values are suffi ciently close to each other, they are indifferent to the decision maker, and if the difference between the criteria values is suffi ciently large, there is no doubt which alternative is better according to that criterion.In between there is an area, in which the decision maker is assumed to hesitate between indifference and strict preference.
The ELECTRE methods have originally been developed by Bernard Roy (1968).Several versions of the ELECTRE method have been presented for different situations: ELECTRE I and IS are designed for selection problems, ELECTRE TRI for sorting problems and ELECTRE II, III and IV for ranking problems (see Roy 1991, Yu 1992).ELECTRE II is an older version, where an abrupt change from indifference to strict preference is assumed instead of pseudo-criteria.The main difference between III and IV is that the relative importance indices for the different criteria are not applied in the latter.The ELECTRE III method that is utilised in this study, can be considered as a non-compensatory model (Rogers and Bruen 1998a).It means that a really bad score of any alternative with respect to any one crite-rion cannot necessarily be compensated for by good scores in other criteria.The PROMETHEE methods were developed in the 1980's (see Brans et al. 1986).
In each case, the problem is formulated with a set of distinct alternatives a i , i = 1,...,n and a set of decision criteria g j , j = 1,...,p so that g j (a i ) represents the performance of alternative i with respect to criterion j.These criteria may be ordinal or even descriptive, on the contrary to most decision-aid methods.The values of these criteria may also contain uncertainty, which can be described either by probability distributions (random variation) or fuzzy zones (uncertainty due to ignorance etc.) (Miettinen and Salminen 1999).
The uncertainty is dealt using pseudo-criteria (e.g.Vincke 1992).This means that two thresholds, namely indifference and preference thresholds, are defi ned.The indifference threshold for criterion j, q j , is a difference beneath which the decision maker is indifferent between two management alternatives a k and a l , i.e. a a g a g a q k l j k j l j The preference threshold for criterion j, p j , is a difference above which the decision maker strongly prefers management alternative a k over a l , i.e.
a a g a g a p k l j k j l j Between these two thresholds there is a zone where the decision maker hesitates between indifference and strong preference, i.e. the zone of weak preference.
a a q g a g a p However, the zone of weak preference does not make sense if the criteria are ordinal or descriptive.In such a case, the preference and indifference thresholds could be set to zero.This means that if one alternative is considered better in ordinal scale, this alternative is strictly preferred, as in ELECTRE II.If some of the criteria are ordinal, this should also be taken into account when the outranking results are calculated: some of the procedures that can be used are based on ordinal, some on cardinal properties of the criteria.
The indifference threshold can be defi ned either with respect to the uncertainty of the criteria values or as a threshold at which the differences become perceptible to decision makers (Rogers and Bruen 1998b).Maystre et al. (1994) defi ned the indifference threshold as the minimum margin of uncertainty and the preference threshold as the maximum margin of uncertainty with respect to different criteria.Thus, the preference threshold implies that there is no doubt that a certain alternative is better than the other.However, there are no right values for the thresholds, or even a right way to defi ne them.

The PROMETHEE Method
In PROMETHEE I and II, the outranking degree Π(a k ,a l ), describing the credibility of the outranking relation that 'alternative a k is better than alternative a l ', for each pair of alternatives (a k ,a l ) is calculated as where F j (a k ,a l ) is the preference function and w j are the relative importance of the different criteria (scaled to add up to one in the formula).The weights can be obtained, for example, by giving scores from 1 to 7 to the criteria, with 1 given to the least important criterion (Hokkanen and Salminen 1994).However, the weights could also be obtained from pairwise comparisons as in the AHP method.
In PROMETHEE outranking method, the threshold values are assumed to be constant (see Salminen et al. 1998).The value of preference function F j (a k ,a l ) for a pair of alternatives a k and a l with respect to criteria j are calculated using thresholds p j and q j as In this formula, the linear threshold function is utilised (Fig. 1a).However, six different forms of the threshold function can be applied, which can be either linear, nonlinear, or a step function (see Brans et al. 1986).The criteria and threshold values together constitute the pseudo-criteria.
The outranking degrees Π are used to calculate for each alternative the leaving fl ow, the entering fl ow and the net fl ow In PROMETHEE I the alternatives are ranked based on both the leaving and entering fl ows.These rankings are then used to calculate a partial preorder, where certain alternatives may remain incomparable.In PROMETHEE II the net fl ow is used, which leads to complete ranking (Hokkanen and Salminen 1997a,b).This ranking method utilises the 'cardinal' properties of the valuations, PROMETHEE I the 'ordinal' properties (Bouyssou andPerny 1992, Bouyssou 1992).

The ELECTRE Method
In ELECTRE III method, a concordance index C(a k ,a l ) for each pair of alternatives is computed, describing the strength of a claim that alternative a k is at least as good as alternative a l , utilising pseudo-criteria in a similar fashion as in PROMETHEE method.The concordance index is calculated as where w j are the relative importance of the different criteria (scaled to add up to one in the formula) and c j (a k ,a l ) is the local concordance index, defi ned as (Fig. 1b) In formula (10), constant threshold values are applied.However, in ELECTRE III, the thresholds may be either constant, proportional to the criterion value or they could be expressed with a linear model as a function of the criterion value (e.g.Rogers and Bruen 1998b).
If the decision were based on the concordance indices of alternatives, weighted with the importance of different criteria, the approach would basically be based on an additive utility function similarly as in PROMETHEE method (see Salminen et al. 1998).Then, the weights of different criteria would represent the substitution rates between the different criteria.
However, in ELECTRE III there is also a socalled veto threshold v j , which is used to compute the discordance index for the alternatives.The discordance index is used to model the degree of incompensation between the criteria.This means that an alternative with a very poor value of any one criterion cannot be chosen irrespective of the values of the other criteria.It also means that the weights of the criteria cannot be interpreted as substitution rates, but they represent votes for the criteria (Miettinen and Salminen 1999).The discordance index is defi ned for each criterion as The discordance indices of different criteria are not aggregated using the weights, since one discordant criterion is suffi cient to discard outranking.In environmental planning the veto threshold is appropriate in a sense that some alternatives are not found acceptable at all (Rogers and Bruen 1998b).The closer the veto threshold v j is to the preference threshold p j , the more important the criterion j can be considered (Roy 1991).
Finally, the degree of outranking is defi ned by S(a k ,a l ) as (Miettinen and Salminen 1999).
In basic ELECTRE III method, a descending (Z 1 ) and ascending (Z 2 ) preorder is constructed using the outranking degrees S. The fi nal partial order Z = Z 1 ∩Z 2 is constructed based on these two complete orders.The preorders Z 1 and Z 2 are constructed using a descending and ascending distillation.In distillation procedure, the ranking is based on the number of alternatives outranked by each alternative minus the number of alterna-tives which outrank it (for details, see Maystre et al. 1994).
In the obtained partial preorder some alternatives may be incomparable, i.e. their performance order cannot be determined.A complete ranking may be obtained using, for example, the 'min' procedure (see Pirlot 1995).In the 'min' procedure, the alternatives are ranked according to the minimum outranking degree of each alternative.The alternative having the highest minimum is ranked fi rst, and so on (see Miettinen and Salminen 1999).The 'min' procedure utilises the ordinal properties of the valuations (Pirlot 1995).In this study, the 'min' procedure is used for ranking the alternatives.
The PROMETHEE II method includes an indifference threshold and a preference threshold, but not a veto threshold as ELECTRE III.Also the method by which the alternatives are ranked differs, and thus, these methods may not always produce similar results.However, given similar thresholds, and veto threshold high enough in ELECTRE III, the outranking degrees produced with these methods are identical (Salminen et al. 1998).
The outranking methods are typically used for group decision making situations.In group decision making, the analyst typically chooses the values for the thresholds, and the decision makers only choose the weight of the criteria.Usually, each decision maker gives his/her own weights, and in the analysis the median or mean values for the weights are used (Roy 1991).However, it is also important to study the effect of the extreme values to the weights in a sensitivity analysis.

Outranking and MAUT in Forest Management
Planning -a Case Study

An Application of HIPRE Program
Pykäläinen et al. (1999) used ideas of multi attribute utility theory -more closely the HIPRE program developed by Hämäläinen and Lauri (1995) -in their application of interactive decision analysis (IDA) on strategic planning of Stateowned natural resources.HIPRE allows the use of a modifi ed MAUT-version of the AHP, which utilises sub-utility functions in the evaluation of choice alternatives, as is shown below.HIPRE program was chosen to be used by the Finnish Forest and Park Service (FPS) governing the case study area.Other MAUT-based methods could also be applied to similar natural resources management problems.In Finland, applications of the standard AHP (Kangas 1994) and HERO heuristic optimization (Kangas et al. 1996) in participatory forestry decision support have been presented.
The IDA application was part of a wider participatory planning project initiated by the FPS in Kainuu, eastern Finland.The function of the IDA was to produce comprehensive decision support for the formulation and selection of a forest strategy.The reader is referred to Pykäläinen et al. (1999) for more detailed information.
Initially, four strategies following different scenarios were formulated in the planning project of Kainuu.The feasibility of land use allocations in general and their implications on producing forest outputs was mapped out by doing this.The impacts of the strategies were measured by numeric criterion variables and they were estimated through planning calculations.So called 'Basic strategy' included the current principles of land use allocation.The 'Business strategy' emphasized economical goals of the FPS in Kainuu.The 'Forest recreation' and 'Nature conservation' strategies emphasized the related goals, respectively.
The IDA was started with a decision hierarchy formulation (Fig. 2).The hierarchy consisted of six levels: the total utility, the parties, the four main criteria for forest management, the subcriteria, the criterion variables, and the alternative forest strategies.The preferences of the parties involved (FPS, one regional and four local working groups including 10-12 interest groups each, and the public) were defi ned and included into planning in a form of an additive utility function: where, U tot is the total utility, w j is the weight of party j, U j is the utility of party j, and n is the number of parties involved.The utility of an individual party was calculated as follows: where m j is the number of criteria of party j, a ij is the relative importance of criterion i, u ij is the sub-utility function of criterion i, and q ij is the quantity that the plan produces or consumes the criterion variable i of party j.
So called sub-utility functions (Fig. 3) defi ned the utilities attained through producing different criterion variables.Each sub-utility function was defi ned with respect to one upper level criterion or sub-criterion.By using the sub-utility functions, the values of criterion variables -measured in their own units (m 3 , monetary units, working years and hectares etc.) -were converted to relative sub-utilities which were scaled on a fi xed interval between 0 and 1 (100%).The production possibilities of the planning area were assumed to be fully explained and transitions from the worst to best values of different criterion variables were assumed to have equal effects on the total utility if the variables were weighted equally.
The sub-utilities were made comparable by counting them with weights a ij which described the mutual importance of the criterion variables when striving for the criteria i.The differences between the worst and the best values of the variables were taken into account when defi ning the weights.The sub-utility functions and the weights of the criterion variables were defi ned by experts.
The weights of the main criteria, instead, were defi ned interactively by the parties.With the interactive computer interface, the participants saw immediately how the current set of weights affected the strategy selection.For example, more weight could be given to the criterion not meeting the requirements of the party, and the consequences of this change could be seen immediately.Interactive defi ning of the weights was continued until the participant accepted the plan.Using tactics in this process was reduced by asking the participants justify their utility functions to the other participants involved.
The weights of the parties were defi ned by the FPS (weights in brackets): FPS (0.5), regional work group (0.250), local work groups (0.125) and the public (0.125).By using these weights, the global weights (priorities) were calculated for the other decision elements in respect to the upper level elements related to them.The global weights of the criteria were: FPS's business revenues (0.310), regional socioeconomical values (0.239), forest recreation values (0.244) and nature conservation values (0.206).
Benefi ts related to FPS's business revenues were measured by using the area of commercial forests, FPS's fi nancial surplus in Kainuu and the change in timber volume in commercial forests.The regional socioeconomical values were measured by the effect on employment (direct and indirect) and FPS's turnover.Forest recreation values were measured by the area of recreational forests, the area of commercial forests with recreational values, the recreation value index and the water quality index.The water quality index, in turn, was measured by the area of clearcut forest stands, the area of fertilised forests and the area of ditch network maintenance.The fourth main criterion was the nature conservation value, measured by the conserved area, the area of   1.
Each strategy was considered to be a feasible one in the beginning of the planning process.However, the Finnish conservation program of old forests was constructed simultaneously with the natural resources strategy of Kainuu.In the conservation program of old forests, the area of conserved forests was to be increased from 28 000 ha to 62 000 ha. Also the landscape ecological planning in the FPS called for restrictions in wood production on certain areas (92 000 ha).As a consequence of this, all the initial strategies were not feasible any more.That is why two new strategies were constructed: 'Mixed 1' strategy and 'Mixed 2' strategy.'Mixed 1' was the 'Basic' strategy including the new nature conservation criteria.The 'Mixed 2' strategy was a modifi ed version of the 'Business' strategy.The priorities of the initial and the mixed strategies are presented in Table 2.
In the sensitivity analysis, the utility function was changed according to the possible changes of the parties' criteria or of the weights between the parties.A non-sensitive solution was assessed to be the best one with a higher degree of certainty compared with a very sensitive solution due to the uncertainty involved in planning.In general, the priorities of the strategies were not sensitive to individual changes in the utility function.

Results Obtained with Outranking Methods
The multi-criteria decision making (MCDM) results were calculated with the two presented outranking methods, PROMETHEE II and ELEC-TRE III (based on 'min' procedure).Because of the differencies in the formulation and pref-erence estimation principles between different methods, it was not possible to formulate exactly the same choice situation with them.Thus, there was no guarantee of calculations refl ecting the same preference information, and the results obtained using different methods were not directly comparable.In order to produce a problem formulation as close as possible to the original HIPRE formulation, the weights for the 17 criteria considered in the outranking process were those obtained with HIPRE analysis.Also, the used threshold values were related to the range of variation in the alternatives, to resemble the scaling in HIPRE.
Using PROMETHEE II, the indifference threshold was fi rst set to zero and the preference threshold was set to be the difference between the best and the worst alternative for each criterion.This means that only alternatives with the same criteria values were considered indifferent, and all the comparisons were in the hesitation zone except for the comparison of the best and the worst alternative with respect to each criterion.In the second case, the indifference threshold was one tenth of the range of variation and the preference threshold was set to half of it.This means that differences less than 10% of the range of variation were considered indifferent and the differences more than 50% of the range of variation were not considered to have a greater effect on the decision than the differences which were 50% of the range of variation.
The best alternative in the analysis proved to be the business strategy.The only difference between the two rankings with different threshold values was that the net fl ows were not exactly the same (Table 3).Compared to the MAUT analysis using HIPRE, the only difference was that the forest recreation and basic strategies changed places.
In ELECTRE method, three different threshold combinations were tested.In the fi rst, the indifference threshold was zero and the preference threshold the range of variation, i.e. the same values as in the fi rst PROMETHEE test.The veto threshold was given a value ten times the range of variation, so that it had no effect on the analysis.In the second test, the indifference threshold was 10%, the preference threshold 50% and the veto threshold 100% of the range of variation.This means that in the comparison of the best and the worst alternative the worst was vetoed with respect to each criterion, otherwise the thresholds were similar to the second PROMETHEE test.In the last test, the veto threshold was set to the 75% of the range of variation with respect to each criterion.
In the fi rst ranking, the order was quite similar to the initial analysis, only the basic and business strategies changed order.This may be explained by the use of 'min' procedure -the performance of business strategy was worse in the worst case, for example with respect to the criterion 'change in the timber volume in commercial forests'.When veto threshold was applied, the rank of the business strategy falled to the last place, and the compromise strategies, e.g. the mixed 1 strategy, rose.When the veto threshold was further lowered, both the basic and the business strategies performed lousy and the nature conservation strategy rose (Table 4).
Using the veto threshold only to the economical criteria, excluding the change in the timber volume, retained the rank obtained also with high veto thresholds.Using the range of variation as the veto threshold for nature conservation criteria raised the mixed and nature conservation as best strategies.The similar veto threshold for recreation criteria raised the basic and forest recreation strategies the best.Consequently, the veto threshold is very powerful tool, by which the importance of the considered criteria can be greatly emphasized.

Discussion
All the methods gave somewhat different results with respect to the rank of the alternatives (see also Zanakis et al. 1998).Especially the results concerning the ELECTRE III method differed from the others.The differences were partly due to the 'min' procedure used, which favours compromise alternatives, partly they were due to the use of veto thresholds.However, the calculations of this study should be taken more as illustrative examples of the use of the methods, and, as mentioned above, the results are not directly comparable as such.The inconsistencies in the results of different methods are due to differencies both in the preference estimation process and in the calculation techniques.Not only the fact that the choice problem formulations did not refl ect the same preference structures caused the differencies in the results, but also the differencies in the manners how preference information is processed in different methods.Also, the interpretation of the criteria weights is different in the methods.In any MCDM situation there is uncertainty concerning not only the values of the criterion variables but also concerning, for example, the weights of the criteria.A sensitivity analysis with respect to the uncertain parameters used in the calculations is thus essential.If the obtained solutions are not sensitive to the parameter values, the analyst may be satisfi ed.In other case it may be that not enough information is available (Vincke 1999).However, taking the uncertainty explicitly into account in the calculations would be even more useful (see e.g.d'Avignon and Vincke 1988).
In most situations it might be useful to utilise several decision-aid methods to the same problem.No single method can be expected to tell the one-and-the-only truth of any natural resource planning problem.If the methods do not agree, the decision makers could be given the solutions from different methods with an explanation why they differ.Then, the decision makers can make the fi nal choice among these alternative solutions (Salminen et al. 1998).In practise, however, this may be diffi cult: each method may require different input and the decision makers may not want to spend that much time pondering the problem.
Outranking methods can be applied to decision problems with a fi nite number of choice alternatives.Thus, the outranking methods are at their best in strategic level decision support.For example, in tactical forest management planning typically with huge number of production programme alternatives, more effi cient optimization procedures are needed.The same basic limitation of the small number of decision alternatives to be compared holds with many MAUT and MAUT-related methods, too.For purposes of tactical planning, HERO heuristic optimization method (Pukkala and Kangas 1993, Kangas et al. 2000) making use of numerical optimization and MAUT techniques, for example, could be applied.
Ability to deal with uncertain and fuzzy information is an indisputable advantage of outranking methods.Same holds with the ability to deal with ordinal and other informal preference statements.A further advantage is that preference estimation procedures of outranking methods are versatile and diverse.For example, veto option gives possibilities to express constraints pragmatically and in an intuitively reasonable way.In usual MAUT methods, multiplicative partial utility functions can serve similar purposes.
Generally taken, outranking methods do not require as much preference information as MAUT-based methods.Furthermore, theoretical assumptions for the information usable in calculations (existence of an a priori utility function, additivity, etc.) are not so demanding as is usual with MAUT methods (Vincke 1992).However, the techniques by which the preference information is dealt with in calculations are rather complicated and hard to explain to nonspecialists.
Easiness to use and understand the method, and interpretability of the results, are important qualities of planning methods applied particularly in participatory planning.When compared to applications of MAUT, diffi culties in understanding and interpreting the calculations and their results are, perhaps, the most crucial defi ciencies of most outranking methods.This problem, especially concerning the veto threshold, has been noted also by the developers of the methods (Vincke 1992).
When applying any decision support method it is crucial to understand the technical roles of each part of the evaluation model, and to take these roles into consideration in all the steps of the planning process.More important than what is the choice of the method applied is to fully understand the method and, in addition, to apply it correctly.For example, in outranking methods the analyst may choose the values of threshold values.Especially when considering the veto threshold, this may give the analyst too much infl uence on the decision problem if he/she does not adhere to the role of adviser.
One interesting topic of future studies is to clarify the interpretations of the parameters used in the choice models applied by different MCDM methods, as well as those of the other parts of the choice models.This knowledge is necessary in the estimation of the models in line with the preferences of the decision-makers, so that the calculations produce priorities or rankings that really refl ect the preferences.A further important topic of future studies is the utilisation of different kind of advisory tools in the estimation of the choice models, such as thematic interviews and techniques of interactive preference modelling.
One problem with many MCDM methods is that rank reversal may occur (e.g.Salminen et al. 1998).It also happens with MAUT-related methods if the priority of one alternative depends on other alternatives.In HIPRE, rank reversal may occur because of the scaling used.This means that adding a new (non-optimal) alternative may change the ranks of the initial alternatives.Rank reversal problem is faced also when outranking methods are applied.Possibility of rank reversal together with the other above mentioned defi ciencies emphasizes the need for sensitivity analysis and interactivity in applying the methods.Interactive approach enables the preferences evolving as new information becomes available through calculations or otherwise.In an interactive approach, any decision model is primarily a technical tool helping the search for the optimal solutions; the aim is not to fully describe the preferences but to fi nd good solutions to complex choice problems.
As a conclusion, outranking methods are worth studying further in natural resources management.They serve alleviation to some tasks which have been observed to be bottlenecks of other decision support methods used in multiple use and participatory natural resources planning.In the method development work, it would be important to pay special attention to the interpretability and understandability of the results as well as of the use of the methods, in order to produce applications more useful in practical natural resources planning.It would also be interesting to try to develop hybrid approaches for the integrated use of different decision support methods, not only hybrids of different outranking methods but also hybrids of outranking, MAUT and numerical optimization.Perhaps that kind of hybrid methods would be applicable in tactical planning, too.

Fig. 1 .
Fig. 1.The preference function F j (a) and the local concordance index c j (b).Note the different x-axis.

Table 1 .
The criterion variables and their global priorities.
commercial forests with conservation values, the change in dead wood volume, the change in the area of old forests and the change in the volume of hardwood.The global priorities of these criteria are presented in Table

Table 2 .
The original HIPRE results.

Table 3 .
The results of PROMETHEE II calculations with two sets of indifference and preference threshold values.

Table 4 .
The results of ELECTRE III calculations using 'min' procedure in ranking with three sets of indifference, preference and veto threshold values.