1 Introduction

In psychology and related fields, it is an everyday practice to use measurements to quantify different constructs, but thinking more broadly, we may find that we are extensively measured even before our birth. Measurement can take many different forms, but measurement in psychology is not always as straightforward as it might seem. In this paper a short summary of the two basic approach of test theory is outlined, and it will also be demonstrated how certain measures of test theory can be obtained in the freely available and very widespread R statistical software (R Core Team, 2021). It is to be emphasized that no basic syntax of the R language is provided here, it is outside the scope of the present paper. It is also has to be noted that in R several ways may be to obtain a certain result, here we show one way, but several other methods may exist to get these or similar outputs.