The sample size for my Knowledge, attitude, and practice survey is 400. From the information from this survey, I would like to develop an educational intervention. To assess the impact of this intervention, I am using the pre-post-intervention design. How could I calculate the sample size for the pilot intervention?
Hello Preeti,
The answer depends on several things.
1. How small a difference do you wish your study to be able to detect, if in fact such a difference actually exists? This is the target effect size (ES). (The smaller the target ES, the larger the requisite sample size, or N of cases.)
2. What probability do you want your study to have of correctly detecting the presence of your target ES? (Higher means larger N. Note that the only way to get a probability of 1.0 is to include the entire population.)
3. What Type I error risk ("alpha level") do you intend to use for the hypothesis test? (Smaller risk means larger N.)
4. What statistical test will you use? If it's something like a regression model, how many variables are involved (IVs and DV/s)? (More variables generally means larger N.)
Given these bits of information, one can pretty handily determine a lower bound sample size using tools such as the freely available g*power program (https://www.psychologie.hhu.de/arbeitsgruppen/allgemeine-psychologie-und-arbeitspsychologie/gpower).
Finally, if your design is a one-group, pre-post design, I'm sure you know that this research design offers very little protection against many of the so-called threats to (study) validity. https://journals.lww.com/epidem/Fulltext/2020/05000/A_Graphical_Catalog_of_Threats_to_Validity_.11.aspx
Good luck with your work.
Hi,
Generally, 12 is the number for surveys in pilot studies. Here are a few references:
Sample size of 12 per group rule of thumb for a pilot study.Steven A. Julious, 2005. Pharmaceutical Statistics.https://doi.org/10.1002/pst.185
Viechtbauer W, Smits L, Kotz D, Budé L, Spigt M, Serroyen J, Crutzen R. A simple formula for the calculation of sample size in pilot studies. J Clin Epidemiol. 2015 Nov;68(11):1375-9. doi: 10.1016/j.jclinepi.2015.04.014
You can use power analysis to calculate the number of participants needed for a pre-post study design. Here are the general steps:
It is important to note that the sample size calculation is based on assumptions about the effect size, significance level, power, and correlation between the pre-test and post-test scores. These assumptions should be based on previous research or clinical experience to ensure that the study has adequate power to detect meaningful differences.
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