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I am carrying out an evaluation for an intervention using secondary data. Unfortunately the design of the study is weak as participants cannot be randomised. I am looking to see whether the intervention has had any impact on patients quality of life? A measure of QoL has been taken before and after the intervention, i am also interested in a number of variables and their potential effect on reported QoL including: age, gender, pharmacological treatment, disease duration, mariatal status, and employment status(re literature). These IV's are both continuous and categorical, but can be all be converted to categorical if needs be (QoL scores will remain continuous). I would appreciate your advice concerning which design is best suited to this piece of research.

I am considering using a ANCOVA but my experience with violations of assumptions may further weaken the merit of an already compromised design.

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Just to note, you may also have the problem of arguing about the presence of response shift. –  King Apr 17 '12 at 10:12
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You can't really evaluate most of the assumptions of ANOVA/ANCOVA/Regression before running the routine. Most of them are about the residuals –  Peter Flom Apr 17 '12 at 10:23
    
You may be able to make inference about the before/after differences. What do you mean by "the design of the study is weak as participants cannot be randomised"? –  Macro Apr 17 '12 at 12:11
    
It's not clear, but do you have a control group who did not receive the intervention? If not, I think you will have greater difficulty in discussing placebo vs. actual effect of intervention. –  TARehman Apr 17 '12 at 17:49

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