Paired T Test

A test used to measure the statistical difference between the means (a.k.a. average) of two continuous (a.k.a. variable or numerical) values. This test is best for dependent variables that are sampled from the same group (e.g., measurements from the same machine before and after a system upgrade, or performance metrics from the same call center before and after process changes, or sales metrics from teh same store before and after new training, etc.). It is generally used when the distribution for the continuous value is normal, hence the use of the mean. When the distribution is non-normal, the mean may not be valid so the median would be a better measure for central tendency; in those cases, try the Mann-Whitney Test. For data having independent variables, use a Two Sample Test.
Related StatStuff Video: 
   -Section 6-Analyze Phase, Lesson #17 - Hypothesis Testing: Central Tendency – Normal (Compare 1:1)