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. 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.
Warm Regards
Dara
Solved – Help with change scores and/or ANCOVA design
ancovachange-scores
Best Answer
QOL should be treated as ordinal in most cases, so the proportional odds model is a good candidate for ANCOVA. It is seldom a good idea to analyze change (you didn't suggest that it was but many do), so baseline QOL can be a covariate in the prop. odds ordinal logistic model. If QOL has only a handful of levels you can represent the baseline variable as categorical with multiple dummy variables.
If treatment was not randomized you may need to aggressively adjust for confounding. If you don't have the sample size to support simultaneous modeling of all potential confounders, the propensity score approach may be used in conjunction with the prop. odds model.