Study design for microgravity human physiology experiments.

In some areas of research, such as microgravity life sciences, both the number of subjects and the opportunity for repeated observation under experimental conditions are limited. Small N study designs are appropriate for these situations since they require few subjects and few observations in the experimental condition. Small N studies compare treatment conditions separately for each subject so that between-subject variability does not obscure treatment effects. Multiple observations are collected for each individual in the baseline condition to ensure a stable reference point for comparison with the smaller number of observations collected under the experimental conditions. Individual differences can be overlooked in large N studies, but in small N studies the focus is on the individual, and differences observed between particular individuals can be suggestive of underlying causal processes. We present a step-by-step approach to designing and conducting a small N study.