A. Hoffmann1, R. Möslein2, A. Scharf 3

1University of Göttingen, Germany, 2isi GmbH Göttingen Germany, 3University of Applied Sciences, Nordhausen, Germany

The implementation of affective sensory tests stands increasingly in the stress ratio between a requested high internal validity on the one hand and a requested stringent external validity on the other. “Internal validity” is given if measured acceptance differences result from the physico-chemical character of the product and all uncontrollable disturbance variables are standardized or controlled. The measured results are considered to possess “external validity” if they can be generalized to the target population of the test and to consumer behaviour.

The aim of the present study is to ascertain whether internal validity is a requirement for external validity. Meaning: if test results shall be generalized to an underlying population the test conditions must be controlled and disturbance variables eliminated. Furthermore it is analyzed if an increase of external validity spirits relative product differences away.

The empirical part was carried out on four coffees. A conducted consumer test, consisting of three different subgroups, provided sensory acceptance data of the different coffee samples. All participants usually prefer coffee with milk or cream. The first group tasted the four coffees with an individual dose of milk or cream illustrating a high degree of external validity. The second subgroup tasted the samples “black” demonstrating a high level of internal validity. The third group served as a control group and tasted the products with a standardized dose of milk.

The research project determines to what extent test results will alter if the experimental design accommodates either exclusively internal or external validity. The test outcome shows interesting effects between the different subgroups: Compared to the “control group” with the standardised amount of milk, the relative acceptance difference between the four coffee samples remain the same when tested “black” whereas all acceptance scores are a little lower in the “black” group which is due to the fact that the respondents usually taste coffee with milk or cream and they are now forced in a situation that does not correspond with their usual behaviour. Interestingly, in the test group where a high level of external validity was maintained, the acceptance differences between the products vanished – meaning that the individual dosage of milk covered the physico-chemical differences which were existent in the “black” and “standardised” situation. With these results, a clear recommendation for internal validity as a “must” for the accomplishment of tests where the primary goal is to measure responses to different variants can be given.

Keywords:
sensory research, internal and external validity, acceptance test, coffee