A confidence interval is a measure of how sure we are about a value.
It is the mean of your estimate plus mean times your confidence number.
proof for later
For a desired margin of error (ME), we can calculate the sample size:
Solving for n:
Always round n to the next whole number.
See also estimation-and-testing, CH4A Lecture Notes.
95% confidence interval:
See also CH4A Lecture Notes.