12.89 df for t tests in regression In regression modeling, for t tests about regression parameters, df = n - number of parameters in equation for the mean.

a. Explain why df = n - 2 for the model my = a + bx.

b. Chapter 8 discussed how to estimate a single mean m.

Treating this as the parameter in a simpler regression model, my = m, with a single parameter, explain why df = n - 1 for inference about a single mean.