The two sample ttest is a hypothesis test used to to compare the means of two reasonably small (30 or less) samples to see if it is feasible that they come from the same population. The hypothesis is:
H_{0} the population means are equal
H_{1} the popular means are different
The test is:
Step 1: calculate the pooled standard deviation:
Step 2: calculate the t statistic:
The degrees of freedom:

sample means 
n_{1}, n_{2} 
sample sizes 
s_{1}, s_{2} 
sample standard deviations 
ν 
degrees of freedom 
See also Paired tTest and One Sample tTest
