35.8 Statistical validity conditions
- The relationship is approximately linear.
- The variation in the response variable is approximately constant for all values of the explanatory variable.
- The sample size is at least 25.
The sample size of 25 is a rough figure here, and some books give other values.
In addition to the statistical validity condition, the test will be
- internally valid if the study was well designed; and
- externally valid if the the sample is a simple random sample from the population.
Example 35.7 (Emergency department patients) In Example 35.4, the scatterplot (Fig. 35.9) shows that the relationship is approximately linear, that the variation in ED patients seems reasonably constant for different numbers of days after distribution, and the sample size is larger than 30.The test (Example. 35.4) and the CI (Example. 35.5) should be statistically valid.