Several method variables were also found to moderate the effect. As discussed earlier in the literature review section, it is necessary to examine the relationship between study quality and outcome (Plonsky, 2011b). One important observation was made on the proficiency standards adopted by primary studies. Studies which combined multiple proficiency scales (e.g., standardized test results, in-house test results, length of language learning, and impressionistic judgment by researchers or instructors) obtained larger effects than those that simply used standardized test scores. This can be interpreted as indicating that combined proficiency scales can reliably place participants into appropriate levels to reflect the real magnitude of the difference between two groups. These findings indicate that it is probably more reliable to use multiple means to determine candidates’ proficiency level. Additionally, the results of research question three suggest that the proficiency variable seems to have effects on peer-peer interaction. This further accentuates the importance of adequately placing candidates into an appropriate proficiency level.