The one-sample t-test is a statistical hypothesis test that can be used to see if an unknown population mean differs from a given value.
Use a paired t-test to compare the group mean over time or after an intervention if you're only studying one group, or a one-sample t-test to compare the group mean to a standard value if you're only studying one group. Use a two-sample t-test if you're comparing two groups. Use a two-tailed test if you only want to know if there is a difference.
To compare two related means, a paired samples t-test is performed. It examines the null hypothesis that the difference between two related means is equal to zero.