We can define confidence interval as a measure of the degree of uncertainty or certainty in a sampling method.

To calculate confidence interval, we use sample data that is, the sample mean and the sample size.

**Formula to calculate confidence interval in excel.**

In excel there are two confidence interval functions,

- The confidence interval for a normal distribution (=CONFIDENCE.NORM)
- Confidence interval when you are using the T’s distribution (=CONFIDENCE.T)

**Example:**

Suppose you are given a significance level or alpha 0.05 and the standard deviation of the population is 2.5 and your sample size is 100.

You will need to transfer the data to an excel spreadsheet, since the sample size is greater than 30, we will use CONFIDENCE.NORM and a table should appear where you will fill in your data as directed by excel.

Click OK and get your answer.

Therefore, your confidence interval is 0.489991.