4.2. Correlation between factorsOne of the problems with multiple regression is that factors may be correlated. For example, temperature is highly correlated with precipitation. If factors are correlated, then it is impossible to separate the effect of different factors. In particular, regression coefficients that indicate the effect of one factor may change when some other factor is added or removed from the model. Stepwise regression helps to evaluate the significance of individual terms in the equation.
First, I will remind you the basics of the analysis of variances (ANOVA) SSR is the sum of squared deviations of predicted values (predicted using regression) from the mean value, and SSE is the sum of squared deviations of actual values from predicted values. The significance of regression is evaluated using Fstatistics:
where The nullhypothesis is that factors has no effect on the dependent variable. If this is true, then the total sum of squares is approximately equally distributed among all degrees of freedom. As a result, the fraction of the sum of squares per one degree of freedom is approximately the same for regression and error terms. Then, the Fstatistic is approximately equal to 1. Now, the question is, how much should the Fstatistic deviate from 1 to reject the null hypothesis. To answer this question we need to look at the distribution of F assuming the null hypothesis: If estimated (empirical) value exceeds the threshold value (which corresponds to the 95% cumulative probability distribution) then the effect of all factors combined is significant. (See tables of threshold values for P = 0.05, 0.01, and 0.001) Note: In some statistical textbooks you can find a twotail Ftest (5% area is partitioned into two 2.5% areas at both, right and left tails of the distribution). This is a wrong method because small F indicates that the regression performs too well (some times suspiciously well). Null hypothesis is not rejected in this case! If F is very small, then we may suspect some cheating in data analysis. For example, this may happen if too many data points were removed as "outliers". However, our objective here is not to test for cheating (we assume no cheating). Thus we use a 1tail Ftest. The Fdistribution depends on the number of degrees of freedom for the numerator [df(SSR)] and denominator [df(SSE)].
Standard regression analysis generally cannot detect the significance of individual factors. The only exception are orthogonal plans in which factors are independent (=not correlated). In most cases, factors are correlated, and thus, a special method called stepwise regression should be used to test the significance of individual factors.
The stepwise regression is a comparison of two regression analyses:
where SSR and SSR1 are regression sum of squares for the full and reduced models, respectively; df(SSR) and df(SSR1) are degrees of freedom for the regression sum of squares in the full and reduced models, respectively; SSE is the error sum of squares for the full model; and df(SSE) is the number of degrees of freedom for the error sum of squares. Because only one factor was removed in the reduced model, df(SSR)  df(SSR1) = 1. The Fstatistic is related to the tstatistic if the denominator has only one degree of freedom:
Thus, the tstatistic can be used instead of the F in the stepwise regression.
Example of the stepwise regression:
