Summing up information from interview or questionnaire data into a single number, an index, is not uncommon in social medical research, but the conceptual and mathematical background for this is too often neglected. The so-called "Rasch models" specify circumstances where a single index can be used. The necessary computer programs for doing this are now generally available (10, 11). Two often used questionnaires concerning neurotoxicity due to solvent exposure are tested using a Rasch model. None of the questionnaires were found to be in agreement with the necessary requirements for the model and no single number describes the respondent's degree of neuropsychiatric dysfunction. Much more work is needed before a suitable screening questionnaire is available.