This paper provides a methodological review of previous research on cognitive task complexity, since the term emerged in 1995, and investigates why much research was more quantitative rather than qualitative. Moreover, it sheds light onto the studies which used the mixed-methods approach and determines which version of the mixed-methods designs was frequently used. The results reveal that many studies in the field of cognitive task-based research used quantitative rather than qualitative experimental design to collect and interpret their findings. These studies were more oriented to post-positivism supported by an objectivist epistemology. However, limited studies utilized mixed-methods approach to consider the effects of cognitive task complexity on linguistic performance in L2 context. The theoretical perspective behind these studies was pragmatism. The mixed-methods studies only used the explanatory sequential design to collect data and interpret their findings while other versions of the mixed-methods research designs were left undefined. To conclude, the paper identifies the current gap in methodology of the studies and offers recommendations for how to obtain more comprehensive and generalizable findings by utilizing other versions of the mixed-methods studies.