Assumptions for the t-test

Normal distribution curve with sigma noted

Note: σ, "sigma", the scale parameter of the normal distribution, also known as the population standard deviation, is easy to see on a picture of a normal curve. Located one σ to the left or right of the normal mean are the two places where the curve changes from convex to concave (the second derivative is zero).

Part of Dr. Doug Elrod's Theory Behind the t-Test Talk