3.4 Constructing Confidence Intervals
The ci() command from the epiDisplay package calculates confidence intervals using an assumed t-distribution. It defaults to a 95% confidence interval, but this can be changed with the option ‘alpha’, as in the second line of code below for a 99% confidence interval. Note that the 99% confidence interval is wider than the 95% confidence interval, as we would expect.
#--- Generate 95% and 99% confidence intervals
ci(bab9$bweight)
## n mean sd se lower95ci upper95ci
## 641 3129 653 25.8 3079 3180
ci(bab9$bweight, alpha = 0.01)
## n mean sd se lower99ci upper99ci
## 641 3129 653 25.8 3063 3196
Note that the implicit calculation performed by the ci() command is: ˉX±ts√n
ˉX represents the sample mean, t the value of the t-distribution for a given number of degrees of freedom and confidence level, s the sample standard deviation, and n the sample size.