Hi! I am having a serious horrible time with an ANCOVA issue, and I would be immensely grateful for any help.
I am writing an evaluation of a research paper, which can be found here:
http://w3.psychology.su.se/staff/mar...h_Med_2004.pdfBasically, they are comparing a) a group of depressed individuals with a group of non-depressed individuals and b) different types of depressed individuals (eg, dysthymia, minor depression etc) with each other an with the non-depressed individuals. They use ANCOVAs. They use gender as a covariate in all these.
Ok, so, problem: Basically, in Field (Discovering statistics... p397-9) and also Miller and Chapman (2001) as cited in Field (
http://homepages.gold.ac.uk/aphome/ancova.pdf), it says 'analysis of covariance cannot tell us how groups would differ if they did not differ on the covariate' and that 'mistakenly, investigators frequently turn to ANCOVA in hopes of 'controlling for' group differences on the covariate' etc etc (both quotes from M&C).
Therefore, they both seem to suggest that you can't use an ancova if there are differences between groups as to the covariate.
Questions:
1) In my Paper 2, it says that gender differs across groups (according to a chi-squared test), but they still entered gender as the covariate. Is this the wrong analysis method to use then??
2) To complicate matters, it goes on about how the covariate must be independent of the treatment effect etc, but I don't exactly have a treatment effect as it's just measuring scores on tests, and so I got confused. Field puts as an example: anxiety is correlated with depression (anxious people tend to be more depressed), so if comparing anxious with non anxious people, the anxious group might be more depressed. Might want to enter depression as covariate to find 'pure' effect of anxiety, but you can't as anxiety and depression share variance. I can understand this example, but unfortunately I can't relate it to my Paper 2 as I am confused. Gender is correlated with depression (or at least, females more likely to be depressed, is that the same thing?)
- does this mean 'gender shares some of its variance with depression'?
Moreover it seems that if gender affects test scores, which it might, then it also can't be used as a covariate..?
Lastly, some papers say you can only use a continuous variable as a covariate, but gender is obviously categorical.
Essentially, have they used ANCOVAs incorrectly, then? What should they have done instead, if so?
REALLY CONFUSED!! Any help would be much much much appreciated.
Isabel