If your research question is "does cancer cause depression and loss of life quality" you should compare non cancer patients with cancer patients ie start with the "exposure = cancer" and compare the exposed group to a non exposed group otherwise similar, This is a cohort design, not a case-control design.

if on the other hand you have a group of patients with depression and poor life quality (the cases) you should select a group without depression or poor life quality (the controls) and compare the presence or absence of cancer in the in the two groups. This would be a case control study..

Ii would recommend the cohort approach.

Dear cancer patients can be your case group and apparently healthy individuals age and sex watched become the control u less you want to come the of depression on cancer patients to other patients with different conditions.

Hello dr Tayseer ,,,

Adding to the above answers. If you don`t know the outcome in the study which are quality of life and depression so it will be cohort that a researcher will calculate incidence and relative risk. This will be known as prof. dr. James answered regarding the hypothesis question. The other issue please noted that researcher will face bias like selection bias and confounding bias if the study design is case control or cohort regarding the answers mentioned above.

** confounding bias like age between smokers and patients with lung cancer. i.e a researcher can`t put the age as an independent variable in cancer group or smokers` group ( A situation in which the effect or association between an exposure and outcome is distorted by the presence of another variable ).

** Until know as I understood : you need to know depression result ( yes or no ) or to be measured by score or scale if present in psychological branch and quality of life by score. >>>>> Then you don`t know the outcome (results) of study >>>> cohort study will be recommended. But researcher need to begin with exposed group and non exposed group related to one disease like cancer (exposed) and non cancer group ( non exposed ). This is a prospective study to check the impact of the cancer.

*** Also which type of cancer. Or cancer group as general depending on that cancers share in common causes.

I completely agree with Dr. Leigh. It appears that you want to do a cohort study, and your most appropriate comparison cohort would be a group of children not known to have cancer or other chronic conditions. If you also think of childhood cancer as one type of chronic condition, then you might have a second comparison cohort of children known to have a chronic condition. That would allow you to calculate the relative risk of depression in children with cancer and the relative risk of depression in children with some other chronic condition and compare those relative risks.

Dr. Leigh also points out to you what it would take to do a case-control study of the issue of depression and childhood cancer. Your cases would be depressed children; and the controls would be children who are not depressed. Then you would have to determine which of the children in each group had cancer. Since cancer is a relatively rare condition among children, you would have to have enormous groups of children with and without depression to do the study, and that would be an inefficient approach.

Dear,

I think that the most important would be to choose controls with diseases that don´t have the same risk factors of the cases (for example, lung cancer cases should not have oral cancer control, as for smoking relevance as risk factor for both), as this would lead to selection bias.

As a broader though, I think that your cases would be depressed children and controls the not depressed ones, and having cancer would be a exposure. I think that this desing should not control by the exposure, but the exposure is not very prevalent, what could make it hard to get enough sample. Case controls are not the best ones for rare exposures, so you may take that into account.

Best wishes,

Abdulwali Sabo

I agree with Mr. Leigh . He has guided in right direction .