Quantitative research synthesis employed statistical methods to synthesize several research papers which were identical in terms of research problems Meta-Analysis in order to discover a standard index that was able to synthesize the aforementioned research papers and to measure the quantifiable effects of manipulated variable on the dependent variable. Regarding this article the author presented and explained Longford’s estimation of effect size which is one of the most efficient and recent estimations. The study proved the background and functional model of the estimator of effect size. In addition the calculation examples following the estimator of Longford’s effect size were demonstrated by meta-analysis.