Paired T Test

The two-sample t-test is used to determine if two population means are equal.
The data may either be paired or not paired.

For paired t test, the data is dependent, i.e. there is a one-to-one correspondence between the values in the two samples.
For example, same subject measured before & after a process change, or same subject measured at different times.

For unpaired t test, the sample sizes for the two samples may or may not be equal.