1. Study design . One factor complicating interpretation is differences in study design. Sample sizes vary, with the majority modest or small. Underpowered studies, combined with potential publication bias, can lead to an increased risk of Type 1 errors [26]. Sampling strategies vary from population-based methods [15,27] to convenience sampling [28,29]. Different ancestral populations have been included, with a preponderance of samples of European ancestry. The age range of subjects varies, and it has been suggested that GxE effects are most consistently replicated in young adult samples [18].
