Degrees of Freedom for Slope T-Test

The slope estimate would become biased upward and the p-value would be inflated, reducing the chance of rejecting $H_0$.

The miscalculation would affect only the width of the confidence interval, leaving the p-value and t-test results unchanged.

There would be no effect on the p-value or conclusions since the difference between $n-1$ and $n-2$ is negligible.

Using $n-1$ degrees of freedom instead of $n-2$ would overestimate the degrees of freedom, likely resulting in a smaller p-value and an increased risk of a Type I error.