In regression modeling, for tests about regression parameters, df = n – number of parameters in the equation for the mean.
a. Explain why df = n – 2 for the model μy = α + βx.
b. Chapter 8 discussed how to estimate a single mean μ. Treating this as the parameter in a simpler regression model, µy = μ, with a single parameter, explain why df = n – 1 for inference about a single mean.
a. df = n – the number of parameters; for the model, µy = α + βx, there are two parameters α and β, and so df = n – 2.
b. When the inference is about a single mean, there is only one parameter, and therefore, df = n – 1.