The question specifically refers to this query: Does the DM establish his preferences from the very beginning or at the end of the model processing data, which is, following a top down, intermediate or bottom up approach?

MCDM is without a doubt an activity with a good deal of subjectivity in certain criteria (for instance, those related with uncertainty, such as data on public opinion or working with an estimated demand), but also with reliable and exact data on others (for instance, tested and approved values for equipment performance). In addition, criteria in type, areas, or fields are usually unknown and must be established, as well as limited in number, mostly for practical restrictions imposed by the MCDM model and work load. To complete the scenario, the DM makes subjective appreciations such as determining weights for criteria, establishing acceptance thresholds, determining preferential type of distances, etc.

Consequently, subjectivity is unavoidable.

The question is: Should subjectivity in the mentioned areas exerted at the very beginning of the process, that is affecting actual values (the top down approach), or is it preferable to run the model with the initial reliable data and apply judgment at the end (the bottom up approach)?

I prefer the second because once reliable or approximate data is processed, there is a result expressed as a ranking, which can be examined, tested, and changed by the DM as per his preferences and judgment and know how.

The DM is in condition to apply his common sense, perception and expertise to modify what he considers is not acceptable for whatever reasons, or that could be improved.  Assume for instance that selecting equipment the ranking is D>A>B>C. This is a purely mathematical approach, and then the DM can say, “OK the best equipment selected by the model is D, but in my opinion I would reverse the ranking and select equipment A. Why? Because I have worked with equipment A and I know it is reliable, sturdy, and with reasonable maintenance costs, while equipment D incorporates a new technology that I don’t know if it has been sufficiently tested”.

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