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Too small a sample is one of the most common problems seen in clinical trials.
Sample
size goes hand-in-hand with power: if no difference between treatments is
seen, it could be because there was not enough power to detect a true
difference.
Even if a study is designed with adequate power to evaluate the primary outcome, the sample size may be inadequate for secondary outcomes or subgroup comparisons. Sample size targets can be calculated using published formulas (see Piantadosi) or may be estimated from tables. |