# Confidence Interval for Mean Calculator for Unknown Standard Deviation

Instructions: Use this step-by-step Confidence Interval for Mean Calculator, with unknown population variance, by providing the sample mean, sample standard deviation and sample size in the form below:

Sample Mean $$(\bar X)$$ =
Sample Standard Deviation $$(s)$$
Sample Size $$(N)$$
Confidence Level (Ex: 0.95, 95, 99, 99%) =

## Confidence Interval for Mean Calculator for Unknown Population Standard Deviation

A confidence interval corresponds to a region in which we are fairly confident that a population parameter is contained by. The population parameter in this case is the population mean $$\mu$$. You need to specify a certain confidence level, which will determine the width of the confidence interval. The following expression is used to compute the confidence interval for the mean:

$CI = \displaystyle \left(\bar X - t_c \times \frac{s}{\sqrt n}, \bar X + t_c \times \frac{s}{\sqrt n} \right)$

where the critical value correspond to critical values associated to the t-distribution with $$df = n - 1$$ degrees of freddom. The critical value for the given $$\alpha$$ and $$df = n - 1$$ is $$t_c = t_{1 - \alpha/2; n-1}$$.

### Assumptions that need to be met

As for most of the confidence intervals we have dealt with, this calculator require that the sample is drawn from a normally distributed population. In this case we don't need the population standard deviation $$\sigma$$ to be known, and we can use instead the sample standard deviation $$s$$.

### Other Calculators you can use

In case the population standard deviation is known, you can use this confidence interval calculator for a population means when the population standard deviation is known .