The interpretation of single individuals' measurements.

This article concerns the interpretation and construction of measurements for single observational units, including the creation of scales or indexes to improve the quality of the measurements. The focus is on the individual as the observational unit in psychology, but to present a broader perspective related measurement issues in official statistics are also discussed. It is concluded that when individual measurements are to be interpreted, measurement precision must be given priority and taken into account in the research design. Unfortunately, most measures in psychology are not highly reliable, and examples are given demonstrating that such measures do not normally allow the researcher to make inferences about single individuals. Methods for testing questionnaires in a cognitive laboratory that have been developed within survey research can provide useful tools to increase both reliability and validity of single questions/items.