An effect size can be defined as a standardized and scale-free measure of the relative size of the effect of an intervention. – Thus, the magnitude of an intervention’s effect or impact (Cohen, 1988; Kline, 2004). Although there are many reasons for using effect sizes, there are two main reasons why effect sizes have become so popular and wide-spread across various sectors:
The use of effect sizes has grown significantly of the past three decades. So much is the perceived value of using effect sizes, that across various disciplines many professional bodies, journal editors and statisticians have mandated its inclusion as necessary in order to clarify and substantiate differences in research findings (for example, American Psychological Association Manual 2001, 2010a; Baugh & Thompson, 2001; Kline, 2004).