Category: Small sample size issues. These pages outline some of the practical issues and ethical concerns with small sample sizes. Articles are arranged by date with the most recent entries at the top. You can find the theme and closely related categories and other resources at the bottom of this page.

Stats: Can I run a quantitative analysis on this data? (June 17, 2008). I get lots of questions about how small a sample size can be before you can't perform a quantitative analysis and instead are forced to summarize the data in a qualitative fashion. The most recent question involved looking at infants with feeding disorders. There were 29 of these infants, but a subgroup of 5 had disorders so severe that they still required a feeding tube at 3 years of age. The researcher wanted to compare this group of 5 to the remaining 24.

Stats: Cohen's Kappa with small cell sizes (April 26, 2007). Someone on Edstat-L wrote in asking about using Cohen' Kappa with a small sample size in some of the cells.

Stats: Perfect isn't quite good enough (December 12, 2006). Someone wanted me to double check their calculations for Fisher's Exact test. If the control group, 3 out of 10 patients experienced an unfortunate outcome. In the treatment group none did (out of 6). You would think that a perfect result in the treatment group would be compelling, but the one-sided p-value for Fisher's Exact test is 0.21.

Stats: Small sample size, yet again (March 29, 2006). Dear Professor Mean, Is there any statistical test/method that will allow you to make statistically significant conclusions from a sample of nine? Someone was trying to tell me that if you use a nonparametric test, you can make get statistical significance, even with a very small sample size.

Stats: When one group only has a single observation (May 24, 2005). Someone asked me about a lab study comparing expression levels for two groups of patients. The first group has two copies of a gene and the second group has three copies of the gene, thanks to a chromosomal duplication. That sounds easy enough to do. You could use a t-test in SPSS. Actually, I prefer to use the general linear model, which provides exactly the same test, but the output looks nicer and it allows you to easily incorporate more complex research designs. The kicker in this analysis, though, is that there is only one patient in the second group. This person asked if he could perform a t-test in SPSS.

Stats: Small sample size (October 11, 2001). Dear Professor Mean, Are there problems with a very small sample? Can the t-test be used with a sample of just three subjects? -- Anxious Abdelwahab

Stats: All or nothing (August 18, 1999). Dear Professor Mean, I would like to know the minimum number of patients needed in order to achieve statistical significance. I am assuming a perfect research situation where all of the patients who got a treatment lived and all the patients who got the placebo died. What would the proper sample size for an all or nothing response be?-- Hesitant Harrison

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This webpage was written by Steve Simon on 2007-09-11, edited by Steve Simon, and was last modified on 2008-07-08. Send feedback to ssimon at cmh dot edu or click on the email link at the top of the page.