To be in a position to decide for himself which course to take, a subject should be offered an evaluation instrument that can be directly interpreted, and with adequate validity and reliability. These two theses are illustrated by the presentation of the QIPOS questionnaire. The first part of the article shows how it was constructed and can now be used. The second illustrates how should be controlled its measuring properties, in this particular decision situation, where no comparisons are to be made between subjects, but only within the individual. A minimal example is presented, dealing with two groups in two dimensions, to show the principle of the generalizability study that should be realized.