PHEPB: Case control studies in cancer research Flashcards

1
Q

Describe the design of a case control study

A

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

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2
Q

What is the difference between casae control and cohort studies?

A

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]

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3
Q

How are controls selected for case control studies?

A

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

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4
Q

What is confounding and what can it lead to?

A

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

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5
Q

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

A

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

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6
Q

What can we not calculate in a case control study?

A

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

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7
Q

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

A
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8
Q

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

A

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

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9
Q

How do we interpret odds ratios?

A
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10
Q

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

A

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

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11
Q

Describe bias in case control studies

A

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

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12
Q

What are the types of error?

A

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

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13
Q

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

A

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

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14
Q

What are historical case control studies?

A

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.

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15
Q

What is reverse causation?

A

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

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16
Q

What is a nested case control study?

A

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.

17
Q

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

A

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

18
Q

Ultimatley, can case control studies conclude causality?

A