Finding critical value $F^*$ given significance level $\alpha$, using the table with critical $F$ values
Assuming a table giving five upper tail probabilities (denoted $p$ or $\alpha$) per df numerator / df denominator combination
Reject the null hypothesis if the observed $F$ falls in the highest $\alpha$ area of the $F$ distribution. In order to find the critical value $F^*$ that corresponds to this upper tail area:
