Was the study big enough? Two café rules

Why is a small study a problem?
When reading an article, we often wonder whether the study was large enough. If a study does not find a statistically significant effect (eg, at p
How can we tell whether the study was too small?

The CI quantifies the error, and thus the uncertainty, associated with the use of study results to draw inferences about wider population values: the upper and lower limits give us the plausible range of population values. If the CI is very wide, then there is little certainty that the study result is a good estimate and the study is likely to have been too small. However, if the CI does not cross the value of clinical significance, then the data are not consistent with a clinically important effect, no matter how large or small the study, how wide or narrow the CI, and how statistically significant the effect. Since studies sometimes do not report a CI, it is helpful to have an approximate idea of the size requirements of different types of studies.