24.8 Conditions for statistical validity
As usual, these results apply under certain conditions. The CI computed above is statistically valid if one of these conditions is true:
- Both sample sizes are at least 25; or
- Either sample size is smaller than 25, and the populations corresponding to both comparison groups have an approximate normal distribution.
The sample size of 25 is a rough figure here, and some books give other values (such as 30). We can explore the histograms of the samples to determine if normality of the populations seems reasonable.
In addition to the statistical validity condition, the CI will be
- internally valid if the study was well designed; and
- externally valid if the sample is a simple random sample and is internally valid.