Figure 7.5: Figure 7.3 Setting the Decision Rule: Two-Tailed, Single-Sample t Test In this sampling distribution of t values (df = 140, α = .05, two-tailed), the border between the rare and common zones is ±1.977. If the observed value of t falls in the rare zone, the null hypothesis is rejected; one fails to reject it if the observed value falls in the common zone. Note that the rare zone is split into two parts, half in each tail of the distribution.