Reference group
All three studies had non-users of HC (previous users + never-users) as the reference group for the main analysis. Study 1 and 3 also made a full analysis with never-users as reference, and study 1 in addition an analysis where each woman was her own control, by comparing the risk of depression during a period before use, with the period after starting using HC, thereby eliminating potential unmeasured confounders between persons if being stable over time (Table 2).