A 90% confidence level means that we would expect 90% of the interval estimates to include the population parameter; a 95% confidence level means that 95% of the intervals would include the parameter; and so on.
Then, what does the 95% confidence interval mean?
If repeated samples were taken and the 95% confidence interval was computed for each sample, 95% of the intervals would contain the population mean. A 95% confidence interval has a 0.95 probability of containing the population mean. 95% of the population distribution is contained in the confidence interval.
How do you find the level of confidence?
To calculate a CI for the population mean (average), under these conditions, do the following:
- Determine the confidence level and find the appropriate z*-value. Refer to the above table.
- Find the sample mean. for the sample size (n).
- Multiply z* times. and divide that by the square root of n.