Reliability Factor, Degree of freedom (df)

Reliability factor
For small samples from a population with unknown variance, reliability factor should be looked up from Student's t table {Look up df=n-1, p=significance/2 = (1-confidence)/2}.

For known variance or for large samples, you can use z-statistic.

Degree of freedom (df)
Used to determine the reliability factor of the confidence interval with the t–distribution. Degrees of freedom = n – 1, where n is the sample size.