See this paper
Grant WB, Boucher BJ. Randomized controlled trials of vitamin D and cancer incidence: A modeling study. PLos One. 2017 May 1;12(5):e0176448. doi: 10.1371/journal.pone.0176448. eCollection 2017. https://doi.org/10.1371/journal.pone.0176448
If your research is observational study you can use the following link
http://www.raosoft.com/samplesize.html
Sekaran (2003) wrote:
Roscoe (1975) proposes the following rules of thumb for determining sample size:
1. Sample sizes larger than 30 and less than 500 are appropriate for most research.
2. Where samples are to be broken into subsamples; (male/females, juniors/seniors, etc.), a minimum sample size of 30 for each category is necessary.
3. In multivariate research (including multiple regression analyses), the sample size should be several times (preferably 10 times or more) as large as the number of variables in the study.
4. For simple experimental research with tight experimental controls (matched pairs, etc.), successful research is possible with samples as small as 10 to 20 in size.
Reference
Sekaran, U., 2003. Research methods for business: A skill building approach. John Wiley & Sons.
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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