How many group variables (samples) do you have?
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More Information (if you need help deciding)
One Sample: If you only have one sample then you must want to compare your sample average to a known population value.
Two Samples: If you have 2 groups you can compare the two groups to see if they are statistically significantly different on your variable of interest.
Many Samples Tests: If you have 3 or more groups you can compare all of the groups to see there are any significant differences, and if there are, which groups are different from which other groups.