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Introduction

Charles Robert Darwin opened The Origin of Species (1859/1958, p. 1), "When we compare .... "Comparison is at the heart of scientific inquiry, and it is central to psychology. The wide variety of methods and designs employed by experimental psychologists is intended to compare individuals and groups or treatments and conditions in order to describe behavior or test hypotheses. The minimum laboratory study in psychology assesses an experimental effect by comparing a condition where a treatment has been applied with one where it has not. Statistical analysis has developed to verify comparisons, hence the psychologist's umbilical connection to it. The overarching significance of comparison to psychology is second nature to investigator and theoretician alike.