A method for the age standardization of test scores

Abstract

A procedure is presented to generate standardized scores from raw test data that are, as far as possible, age independent and normally distributed. The model is fitted to the percentile points of the raw score distribution, and assumes a linear trend of each percentile with age. The fitted slopes can be constant or can vary quadratically with the percentiles. A nonlinear transformation of the data is also possible to allow for "ceiling effects." These models are described and the methods used to fit them to test data are discussed; examples are presented of their use in standardizing tests, and the use of the diagnostic plots produced by the program are discussed. Index terms: age standardization, linear regression, nonlinear regression, nonparallel regression, parallel linear regression, percentiles, score transformation.