How to calculate sample size if the study is proposed to be done in multiple phases, phase 1 qualitative study, phase 2 cross sectional study and phase 3 clinical trial? can we take 1 sample size and continue with it along the entire study?
The (unfortunate) answer is that it will depend on what the intended outcomes and goals of the phases are. For example, a "qualitative" phase might comprise one or more focus groups. The goals here are likely to differ from the clinical trial phase. If the cross-sectional phase is a "baseline" description of the subjects entering the clinical trial, then goals of the clinical trial will likely determine the sample size of the "cross-sectional" or baseline description of the sample. However, if your goal of the cross-sectional study is to estimate the prevalence(s) of one or more characteristics in a representative sample of a target, then a separate sample size estimate might well be necessary.
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Please let me know if the references/sites below are helpful to you:
1. How to Calculate Sample Size for Different Study Designs in ...
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3775042/One of the important issues faced by a biomedical researcher during the design phase of the study is sample size calculation. Various studies published in ...
2. Sample Size Calculation in Clinical Trials - NCBI - NIH
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2933537/Aug 9, 2010 ... In this article, we discuss the purpose of sample size calculation in clinical trials, ... every clinical and epidemiological study and sample size calculation is an essential ... i.e., we do not take into account the problem of multiple tests. .... planning phase, this usually leads to an excessively small sample size.
Dennis
Dennis Mazur
Sample sizes in clinical trials are different from one phase to another. We don’t have any rule to calculate sample size for phase I. However, it must be small. Sample sizes for phase II and III are calculated based on p value (treatment), levels of significance α and power.
I recommend some references
Khan et al. Smaller sample sizes for phase II trials based on exact tests with actual error rates by trading-off their nominal levels of significance and power. Br J Cancer. 2012 Nov 20; 107(11): 1801–1809.
Charan et al. How to Calculate Sample Size for Different Study Designs in Medical Research? Indian J Psychol Med. 2013 Apr-Jun; 35(2): 121–126.
Then how can someone calculate sample size for doing a cross sectional study on patients with a rare disease in the center for that rare disease?
Dear RG colleague,
Determining the sample sizes involve resource and statistical issues. Usually, researchers regard 100 participants as the minimum sample size when the population is large. However, In most studies the sample size is determined effectively by two factors: (1) the nature of data analysis proposed and (2) estimated response rate.
For example, if you plan to use a linear regression a sample size of 50+ 8K is required, where K is the number of predictors. Some researchers believes it is desirable to have at least 10 respondents for each item being tested in a factor analysis, Further, up to 300 responses is not unusual for Likert scale development according to other researchers.
Another method of calculating the required sample size is using the Power and Sample size program (www.power-analysis.com).
Regards,
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