SMALL TRIAL BIAS:
A bias that occurs when one makes decisions about the quality of study based solely on the fact that the size of the sample is small, without evaluating other characteristics1. Such that, if one prefers smaller studies, they may overemphasize the importance of the results if the sample size is small, or if one generally dislikes small studies, they may deemphasize the importance of the results if the size of the sample is small1, and/or focus on the weaknesses of the study instead of its strengths.
Small Trial Bias is a specific type of Interpretive Bias, is similar to Large Trial Bias, and is applicable to all study designs, not just clinical trials as the name indicates. Small Trial Bias is a problem, as small studies are not necessarily more or less valid compared to ones that are large; since sample size and study quality/validity are different concepts. Sample size is related to issues of statistical significance only, and not clinical meaningfulness; both are different and important for assessing study quality and validity.
Arguably, small is a subjective term, thus small should be quantified in terms of number of participants. Clearly, it may be difficult to draw causal conclusions from studies with less than thirty participants, however these studies may still be classified as valid in some contexts (e.g. a case study (sample size of one) may be high quality and valid for generating discussion on a new, or emerging disease, but not valid for drawing cause-effect conclusions). Also see: Interpretive Bias, Large Trial Bias, Significance Bias, and Wrong Sample Size Bias.
Reference:
1. Earl-Slater A. The Handbook of Clinical Trials and Other Research. Abingdon: Radcliffe Medical Press Ltd. 2002. (Link to Reference)