Confidence Interval Math
When determining the confidence interval for a sample of data, 6 values are needed:
This is a portion of a t-table from which the t-value can be found using the degrees of freedom (df) and the confidence interval (i.e. Use the 0.10 column for a 90% confidence interval and the row that correlates with each respective degrees of freedom).
Statistical Methods to Find the Confidence Interval:
Take a random sample from a population
Find the mean of the sample (M)
Solve for s and sm using the equations above
Find the t-value using the t-table which can be found on several sites on the Internet.