Degrees of freedom (df) are the number of independent values or quantities which can be assigned to a statistical distribution. In comparing two population means, df impact the critical value in hypothesis testing.
T-Distribution: A probability distribution used in statistical analyses when sample size is small and population standard deviation is unknown.
Pooled Variance: An estimate of variance that combines data from two or more samples, used when those samples are assumed to come from populations with equal variances.
Critical Value: A point on a statistical distribution that represents an extreme value; it determines whether to reject the null hypothesis.