Can I do an analysis of covariance using regression?
You can run an ancova using regression. Just put the group and covariates in as independent variables. Your regression estimate and its standard error, for the group term, is the difference between the two group means adjusted for the covariates (which SPSS calls 'Estimated Marginal Means'). 'Marginal' is used because group means (e.g. for males and females) are computed pooling across the covariate (using the overall age mean). We remove age differences and end up with (an age pooled one-way layout of) group means which are akin to the marginal table means of a higher order (age by group two-way) table. Chapter 7 of Boniface gives illustrations of computing ANCOVA adjusted means.
You can get out this regression estimate alternatively in SPSS using analyze:General linear Model:univariate:options.
Put the group factor in the display means box (top right) and click the compare main effect box directly underneath and run the ancova as normal.
You also get the bonus of a 95% Confidence interval for the adjusted difference in the group means.
For three or more groups you have to enter them as dummy variables into the regression.
- The GLM Univariate method, on the other hand, will create and fit these dummy variables all for you so saving you the effort of doing a regression.
Reference
Boniface D. R. (1995). Experiment design and statistical methods for behavioural and social research. Chapman and Hall:London.