Selection Effects

Introduction

Selection effects arise when individuals within a studies only are include individuals that are based on some eligibility criterion impacts the distributions of random variables. Range restriction is a selection effect where eligible individuals show less variation in the study population than in the target population of interest. On the other hand, range enhancement describes a selection effect where eligible individuals show more variation in the study population than in the target population of interest. In this book we will cover two broad selection cases: direct selection and indirect selection. Direct selection occurs when individual eligibility is based on the random variables used in the analysis (e.g., eligible individuals are above the mean of \(X\)), whereas indirect selection occurs when eligibility is based on some other variables.