So you want to test if the mean of a numeric variable from a single population equals some claimed value. Question: What is the shape of your variable’s distribution?
A normal distribution is the classic bell curve. It is symmetric, with most cases falling near the mean and fewer cases falling away from the mean in either tail.
Example:
For more information: Assessing normality
If the variable’s distribution is skewed (has a tail in either direction), has more than one mode (peak), or contains outliers, then it will fail the normality assumption of many common statistical methods.
Example:
