Final answer:
In a paired test where each of the 18 individuals has a before and after value, the sample size is 18. These form 18 matched pairs, which are analyzed with the paired samples t-test with 17 degrees of freedom.
Step-by-step explanation:
For a paired test involving comparisons of before and after values in a study, the sample size is the number of matched pairs. In the context provided, where there are 18 individuals and each individual provides both a before and an after value, the sample size is 18. These before and after measurements form 18 pairs, with each pair consisting of the measurements from one individual. The paired samples t-test, also known as the matched pairs t-test or dependent samples t-test, utilizes the differences within each pair to determine if there is a statistically significant change.
To perform this test, one might calculate the difference between each individual's before and after values, after which a t-test for a single population mean with n - 1 degrees of freedom would be conducted, where n is the number of pairs, which in this case is 18.