Question

Explain how to choose the distribution (normal or t-distribution) in the margin of error when you construct the confidence intervals, for continuous outcome and dichotomous outcome, respectively.

Answer 1

For continuous outcome -

• We use normal distribution,
  • If we know the population standard deviation of the variable and the sample size is at least 30.
  • If we know the population standard deviation of the variable and the original population is normal
• We use normal distribution,
  • If we do not know the population standard deviation of the variable and the sample size is at least 30.
  • If we do not know the population standard deviation of the variable and the original population is normal.

For dichotomous outcome, we usually estimate proportion or rate of dichotomous outcome, e.g., prevalence, cumulative incidence, incidence rate. For proportion or rate, we always use normal distribution.