Estimated Values after ANCOVA in SPSS

css

New Member
Hi,
I am conducting a one-way ANCOVA with a fixed factor (group) and a continuous co-variate (height). I compared the estimated marginal means with the means for saved, predicted values, and I found that the sets of means are not equal. In http://www-01.ibm.com/support/docview.wss?uid=swg21477021, explains that this occurs because covariate-adjusted means for all groups are calculated using the grand mean value(s) of the covariate(s) . However, predicted values for individual cases use each case's actual covariate value(s).

My question is: How could I obtain "estimated/ predicted individual values" using the grand mean?

Stats fan

Member
The estimated marginal mean for a group is both the estimated population mean and the predicted value for people in that group, given that the covariate is equal to the grand mean.

css

New Member
Hi Stats fan,

I have then a question, that perhaps is very naif. However, I prefer to look as a fool than to remain ignorant ;-))

if the estimated marginal mean for a group is both the estimated population mean and the predicted value for ALL the people in that group... why there is variability in the estimated marginal mean (e.g. SD)?

Stats fan

Member
The estimated marginal mean for a group is the estimated population mean and the predicted value for ALL people in that group, given that they score average on the covariate. So for someone who scores average on the covariate and is in group 1, the predicted score is equal to the estimated marginal mean in group 1.

If you you are interested in the predicted scores based on the actual score on the covariate of each subject, you should look at the predicted scores that you found via the 'save predicted values' option.

Given a certain value on the covariate and the group number, you will always find the same predicted score. However, the observed scores of two subjects in the same group with the same value for the covariate do not need to be the same. So even amongst subjects in the same group and with the same value for the covariate, there can be variability in the observed scores.

The reason for the 'variability in the estimated marginal mean' (do you mean the standard error?) is that the estimated marginal mean that you found in your sample is just an estimate based on a single sample. In a different sample you will probably find a different estimated marginal mean. The standard error of the estimated marginal mean is the (estimated) standard deviation of the estimated marginal means we would find in an infinite number of different samples.