Any number you want: the impact of data cleaning on internal validity

Abstract

Concerns about the internal validity of reconviction studies tend to focus on factors such as initial comparability of groups. Often overlooked is the impact that data preparation can have. Data preparation refers to the decisions taken by researchers regarding which offenders to retain in the sample for analysis. Using data relating to a sample of offenders in two police forces, it is shown that these decisions, even when applied equally to both groups, can impact differentially on reconviction rates, weakening a study s internal validity. Implications of the findings are considered and recommendations made to improve the transparency of the process