The confidence interval is a way to report the most probable value for a population’s mean μ, when the population’s standard deviation, δ is known. Confidence intervals can be quoted for any desired probability level, several samples of which are shown in the table below.
|z||Confidence interval (%)|
There are 3 methods for calculating confidence interval and their application is dependent on the type of data provided.
Method 1: Confidence interval (CI) given population mean and variance
What is the 95% confidence interval for the amount of aspirin in a single analgesic tablet drawn from a population where μ is 250mg and δ² is 25?
Xi = μ ± zδ
z=1.96(the value corresponding to 95% on the table above)
Xi= μ ± 1.96δ
250mg ± (1.96)(5)
=250mg ± 10mg