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effect sizethe size of effect which is found in a quantitative study. Different types of research design produce different effect sizes; for example, in correlational studies the size ofr (the correlation coefficient) gives an impression of the strength of the association between two variables. Achieving statistical significance is dependent on the STATISTICAL POWER of the statistical test employed. Statistical power is largely dependent on two factors: the effect size and the sample size. Thus a small correlation could be found to be significant when a large sample has been employed, whereas a large correlation could be found not to be significant because a small sample was employed. Conversely, the effect size from a study is largely unaffected by the sample size. Accordingly, statistical significance is not a measure of the magnitude of a result and if researchers wish to compare studies then effect size should be employed. An additional role of effect size is in the synthesis of the results from a number of studies using META-ANALYSIS.
Collins Dictionary of Sociology, 3rd ed. © HarperCollins Publishers 2000