The formula for 95% confidence interval (CI) when the sample size is greater than 30 is
95% CI = mean ± z_score*standard deviation for the sample/square_root(sample size) ... (1)
z-score for 95% confidence interval is 1.96. It is recommended to use t-score instead of z-score if the sample size is less than 30.
95% CI = mean ± t_score*standard deviation for the sample/square_root(sample size) ... (2)
t-table is available on the internet. You need to use Degrees of freedom (df =sample size-1) to find the value of the t-score for the given sample size.
E.g., t-score for a 95% confidence interval if the sample size (n) is 20 = 2.093.
Let's say you have data in column A from A1:A20.
- You can compute the mean in LibreOffice Calc using the formula
- Compute the ratio of standard deviation and the square root of the sample size using the following formula.
Once you have the above two values, you can use them to find 95% confidence interval for the given sample using the formula 1 or 2, depending on the sample size.