T-distribution

The T-distribution can be used instead of the Normal distribution when defining confidence intervals when sample size is small or the standard deviation is unknown. In comparison with the normal distribution the shape is lower and wider to allow for more uncertainty in the edges:

The width and height of the student t distribution varies with number of Degrees of freedom. Lookup values in Student-T distribution table

Part of M7-Quantitative Decision Methods-S3 - Normal Distribution and confidence intervals