Where t α is the critical ratio to determine the probability of a false-positive (α) error if the null hypothesis is rejected is the mean difference that was observed in the outcome variable, s d is the standard deviation of the before-after differences, and N is the sample size. ![]() To derive the basic formula for calculating the sample size, it is easiest to start with the formula for the paired t-test (see Chapter 10 ): As a test of intuition regarding sample size, try to answer the following three questions:Ī Derivation of Basic Sample Size Formula Sample size calculations can be confusing, even for people who can do ordinary statistical analyses without trouble. Statisticians are probably consulted more often because an investigator wants to know the sample size needed for a study than for any other reason. In research reported in the literature, inadequate sample size may explain why apparently useful clinical results are not statistically significant. ![]() Part of their task when reviewing the sample size is to determine whether the proposed research is realistic (e.g., whether adequate participants are included in the intervention and control groups in a randomized clinical trial, or in the groups of cases and controls in a case-control study). ![]() Members of committees responsible for evaluating and funding clinical studies look closely at the assumptions used to estimate the number of study participants needed and at the way in which calculations of sample size were performed. The sample size has a profound impact on the likelihood of finding statistical significance. The determination of sample size is critical in planning clinical research because sample size is usually the most important factor determining the time and funding necessary to perform the research.
0 Comments
Leave a Reply. |