Given: \(N=\text{Sample size}=48 a=\text{Significance level}=0.05\)

The degrees of freedom is the sample size N decreased by 1. \(df=N-1=48-1=47\)

Determine the t-value by looking in the row starting with decrees of freedom \(df=n-1=20-1=19\) and in the column with \(\frac{a}{2}=0.025\) in the table of the Students T distribution in the appendix: \(\frac{ta}{2}=\pm2.01174\)

The common zone is then between the two critical values, while the remaining area is the rare zone.