Suppose that the screening test is based on analysis of a blood sample taken from women early in pregnancy. For example, suppose a study is proposed to evaluate a new screening test for Down Syndrome. However, in many studies, the sample size is determined by financial or logistical constraints. The formulas presented here generate estimates of the necessary sample size(s) required based on statistical criteria. Studies that are much larger than they need to be to answer the research questions are also wasteful. These situations can also be viewed as unethical as participants may have been put at risk as part of a study that was unable to answer an important question. Studies that have either an inadequate number of participants or an excessively large number of participants are both wasteful in terms of participant and investigator time, resources to conduct the assessments, analytic efforts and so on.
Studies should be designed to include a sufficient number of participants to adequately address the research question. This module will focus on formulas that can be used to estimate the sample size needed to produce a confidence interval estimate with a specified margin of error (precision) or to ensure that a test of hypothesis has a high probability of detecting a meaningful difference in the parameter. Boston Univeristy School of Public HealthĪ critically important aspect of any study is determining the appropriate sample size to answer the research question.
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