Evaluation of intervention measures – a methodological approach

It is common for researchers to evaluate interventions by having the participants answer standardized questions before and after, and measuring the effect of the intervention as a statistical difference between the two. There is a risk that the results of such a calculation will be misleading due to the subject interpreting the response scales differently on each occasion. This risk has previously been observed by Golembiewski et al, who indicated that we are dealing with a psychometric problem set that is especially evident in interventions. The question is whether the individual’s own yardstick for assessing or valuing a certain circumstance/area is the same before and after the intervention, or whether the yardstick has changed due to the subject gaining new experience within the conceptual framework. Golembiewski calls this phenomenon beta change. The purpose of this paper is to illustrate empirically the occurrence of beta change and its possible consequences for the interpretation of the results, with the aid of a model devised by Terborg et al. The material is taken from a health-promoting intervention project in the mail distribution division of the Swedish Post Office.