![]() ![]() Thus, the model can be formulated as T i = θ + e i, the sampling errors, e i, being normally distributed with mean 0 and sampling variance σ i 2, e i ∼ N ( 0, σ i 2 ). Suppose there are k independent empirical studies about a given topic and T i is the effect-size estimate obtained in the ith study (here T i refers to any of the different effect-size indices presented above, both from the d and the r families.) In a fixed-effects model, it is assumed that all of the effect-size estimates come from a population with a common parametric effect size, θ, and as a consequence the only error source is that produced by sampling error, e i. To accomplish the first purpose in a meta-analysis, that is, to calculate an average effect size, two statistical models can be assumed: the fixed- and the random-effects models. ![]() The main task of the statistical model is to establish the properties of the effect-size population from which the individual effect-size estimates have been selected. The statistical analyses in a meta-analysis are guided by a statistical model that must be previously assumed. Marín-Martínez, in International Encyclopedia of Education (Third Edition), 2010 Calculating an Average Effect Size ![]()
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