# Compare three groups with very small sample size

I have three treatments: control, treatment1, and treatment2. A total of four patients were examined after receiving each of these three treatments. The dependent variable is continuous. My dataset looks like this:

Patients   control  treatment1  treatment2
patient1    23.4    34.5         67.4
patient2    10.9    78.9         23.0
patient3    23.2    50.8        100.8
patient4    24.2    67.1         90.8


I first performed one-way ANOVA, and got a p-value of 0.45. Then, considering repeated measures data in my case (paired data for each patient), I was thinking that I should use repeated measures ANOVA, but I am having trouble in deciding which test to use for small-sample data.

I hope somebody could provide me not only with a good idea about this specific case, but also about important issues related to small-sample data analysis.

• How can a single patient be in both a treatment and a control group? Commented Dec 22, 2016 at 15:44
• @YuvalSp as long as it's reasonable to assume that measurement under one condition won't effect measurement on subsequent conditions, then it's completely reasonable to do a within-subjects condition manipulation. For example, measure participant's blood sugar on day 1 on an empty stomach (control), on day 2 30 minutes after a carb-heavy meal (treatment 1), and then on day 3 30 minutes after a protein-heavy meal (treatment 2). Commented Dec 22, 2016 at 17:45
• It is actually a control group and both treatment groups. This is possible in a crossover design. I think it would help to know what the response is. I think it is clear that this sample size is too small to compare 2 treatments and a control. The crossover trial is very complicated. The patients may need to wait after one treatment so it won't confound the results with the next treatment. Also we would expect that there would be a fixed time after treatment for the response to be measured. Commented Dec 22, 2016 at 17:46
• @MichaelChernick Normally how you analyze the data from crossover trial similar to my case? Can we just perform repeated measurement analysis to such data? But still the very small sample size is big headache. Commented Apr 6, 2017 at 17:54
• @Amy Have you already collected the data? If so the cross-over design is out of play. Commented Apr 6, 2017 at 18:06