Types of Analysis: Planned (prespecified) vs Post Hoc, Primary vs Secondary, Hypothesis-driven vs Exploratory,...

In research, there are different, overlapping ways in which the plan of analysis may be described. This article explains planned (prespecified) vs post hoc, primary vs secondary, hypothesis-driven vs exploratory, and subgroup and sensitivity analyses; intent-to-treat vs per-protocol vs completer analysis was explained in an earlier article in this column. A prespecified analysis is one that is outlined before the study starts; it is usually separated into primary and secondary analyses for the single primary outcome and the many secondary outcomes, respectively. Exploratory analyses examine relationships between variables in the absence of well-defined expectations; these tend to be statistical fishing expeditions. Subgroup analyses examine findings in categories of subjects in the sample. Sensitivity analyses examine whether the findings remain similar if the data are handled in a way that differs from the original plan of analysis. Post hoc analyses examine hypotheses that are conceptualized after the data are seen. All these terms are explained with the help of examples, and strengths and limitations are briefly discussed.