PHEPB: Case control studies in cancer research

Question 1

Describe the design of a case control study

Answer 1

Case study → group of individuals that actually have the disease
Case control → those who do not have the disease
Compare the 2 to past exposure and postential risk factors - this is why it is a retrospective study

Question 2

What is the difference between casae control and cohort studies?

Answer 2

Cohort → you are looking at incidence - you are lookimg forward in time [Prospective study]

Case control → you already have people w disease - the control should not have the disease but should be comparable - you want them to be similar though (like age, sex, socioeconmic status) [Retrospective study]

Question 3

How are controls selected for case control studies?

Answer 3

Controls must not have the disease but are otherwise comparable to cases. Main sources inc:
General population
-Random sample from the pop that gave rise to the cases.
-Not always practical.
-Subjects non-co-operative, low response rate.
Hospital
-Readily available, time to spare, far more co-operative.
-Controls must not have a condition that has causes in common with the study disease- eg do not use coronary patients in a study about lung cancer as they both have RF in common like smoking

Question 4

What is confounding and what can it lead to?

Answer 4

Does H pylori cause stomach cancer, or is it that age is cuases both stomach cancer and h pylori?? (Age= common confounder)

Question 5

How does matching adjust for confounding factors and what are its disadvantages?

Answer 5

Collect info on potential confounders and adjust for them at the analysis stage. And/or take account at the design stage by matching:
Each case is randomly matched to 1 or more controls (e.g. matched for age and sex). Not always practical.
As you increase the number of matching variables it becomes more difficult to find a match – loss of cases.
Can introduce bias due to over matching. This occurs if the matching variable is on the causal pathway between exposure and disease

Question 6

What can we not calculate in a case control study?

Answer 6

CANNOT CALCULATE RELATIVE RISK IN CASE CONTROL STUDY! Instead, we estimate odds
“did you smoke prior to diganosis” - pt vs “ did you smoke prior to this date” - control

Question 7

This is data on a study for lung cancer and cigarette smoking
Use this table to estimate the odds ratio, and otutline the step by step of how you do this

Answer 7

Question 8

Here the odds ratio is 9.08. Work out the statistical significance of this AND the expected frequencies

Answer 8

Null hypothesis: if there is no differences in cases and controls, then the true odds ratio will equal 1

Expected frequencies= values in the table if we expected the null hypothesis to be true
- yes - case: (1357 x 2646) / 2714 = 1323
- no - case: (1357 x 2646) / 2714 = 1323
- yes - control: (1357 x 68) / 2714 = 34
- no - control: (1357 x 68) / 2714 = 34

Question 9

How do we interpret odds ratios?

Answer 9

Question 10

Dose response can emphasise an association between the variable being tested in case-control studies. Use the data below to calculate the odds ratios

Answer 10

e.g. in the 5-14 category ⇒ 489 x61 / 570 x 7 = 7.48 odds ratio
in the 15-24 category ⇒ 475 x61 / 431 x 7= 9.60

Question 11

Describe bias in case control studies

Answer 11

In designing and analysing a case-control study we aim to ensure:
Comparisons are not explained by confounding.
Comparisons are not explained by bias (systematic error).
Confounding and bias are particularly likely in a case control study
We really do not want a bias in the estimate: odds ratio → this could be a bias leading to systematic error

Question 12

What are the types of error?

Answer 12

If exposure variable is measured w a random error, then it is kinda okay but not really
But
We really do not want a bias in the estimate: odds ratio → this could be a bias leading to systematic error
Even random error can cause bias → this is called bias towards the null - this means that our estimate to odds ratio will be closer to 1 than when it should be

Question 13

What are the different sources of bias in case control studies?

Answer 13

Selection Bias: If the control group is not comparable (e.g. if older)
Methods of data collection:
Information bias: If the way we collect info from cases and controls differs (e.g. If case is too ill family gives info instead).
Random misclassification error: Errors in classification of exposure status occur to a similar extent in both cases and controls (generally biases associations towards the null).
Recall bias: Recall of past info may differ between cases and controls.
Interviewer bias. Must ask questions in a standardised form

Question 14

What are historical case control studies?

Answer 14

historical case-control studies: In some case-control studies exposure info obtained from historical records
Advantage: info recorded prior to disease onset (may reduce the possibility of reverse causation; reduce recall bias)
Disadvantages: Records may be incomplete (loss of cases / loss of controls).
Those extracting the data may introduce bias unless ‘blind’ to case-control status.

Question 15

What is reverse causation?

Answer 15

Reverse causation → in response to early symptoms, they may change their behaviour - this change in behaviour may be mistaken for causing the disease rather than the idea that it is just an effect of the disease

Question 16

What is a nested case control study?

Answer 16

Both the cases and controls are selected from an
established cohort study.
Cases: study subjects who develop the disease of interest over a given period.
Controls: typically each case is matched to one or more cohort members who were disease free at the
time of the case diagnosis.

Question 17

What are the strengths and limitations of a nested case control study?

Answer 17

Strengths
Access to exposure data collected prior to disease onset (reduces the problem of reverse causation;
reduces bias, particularly recall bias).
There is an obvious source of controls (careful consideration still needs to be given to matching and
inclusion/exclusion criteria).
Limits the use of stored biological samples collected as part of the cohort study.
Limitations
Reduced sample size
Limitations of the cohort study

Question 18

Ultimatley, can case control studies conclude causality?

Answer 18