Evaluating Causation Flashcards by Will Perchard

Definitive groups in case-control studies

Cases
Controls

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1st column in table

Exposed
Non exposed
Total

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2nd column

Cases
A
C
A+Cd

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3rd column

Controls
B
D
B+D

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4th column

A+B
C+D
A+B+C+D

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1st row

Cases, Controls

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2nd row

Exposed, A, B, A+B

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3rd Row

Non exposed, C, D, C+D

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4th row

Total, A+C, B+D, A+B+C+D

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Case exposure rate

A/A+C

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Control exposure rate

B/B+D

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Odds of exposure in cases

A/C

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Odds of exposure in controls

B/D

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What is the odds ratio?

Odds of being exposed if you are a case rather than a control

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Calculation for odds ratio?

AD/BC

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At what point does odds ratio = relative risk?

If disease is rare i.e. prevalence <5%

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Values for odds ratio

0 to infinity

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How does one calculate CI from odds ratio

Using log transformations

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What can we calculate from cohort studies?

Risk rates

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Why can risk not be calculated from case-control studies?

Risk = incidence, which needs a prospective observation

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1st column in cohort studies

Exposed cohort
Non exposed
Total

2nd column in cohort studies

Disease develops
A
C
A+C

3rd column in cohort studies

No disease
B
D
B+D

4th column in cohort studies

A+B
C+D
A+B+C+D

1st row in cohort studies

Disease develops, No disease

2nd row in cohort studies

Exposed cohort, A, B, A+B

3rd row in cohort studies

Non exposed, C, D, C+D

4th row in cohort studies

Total, A+C, B+D, A+B+C+D

Absolute risk of disease in exposed

A/A+B

Absolute risk of disease in non-exposed

C/C+D

Odds of finding disease among exposed

A/B

Odds of finding disease in non-exposed

C/D

Odds ratio for cohort studies

AD/BC I.e. same as that for case-control studies

What is the risk difference?

Difference between two absolute risks (exposed and non-exposed)

Formula for risk difference

Absolute risk in exposed group - absolute risk in non-exposed group

Another name for risk difference?

Absolute risk reduction Attributable risk

Calculation of Relative risk

[A/(A+B)] / [C(C+D)]

What is the relative risk?

Ratio between the probability of developing the disease if exposed to the probability of developing the disease if not exposed

Another name for relative risk

Risk ratio

Calculation of population attributable risk (PAR)?

Absolute risk reduction x proportion exposed

What does PAR tell us?

Number of cases that could be eliminated by eradicating the exposure

What is the rare disease assumption?

When the probability of an event is low (<5% prevalence), odds ratio will = relative risk

What happens to odds ratio as prevalence increases?

Odds ratio increases

When are odds ratio used?

Case control studies Cohort studies Analysis of dichotomous outcomes in clinical trials

Range of values of probabilities

0-1

Range of values of absolute risk

0-1 0 = no one in group has disease 1 = everyone in group has disease

Range of values of attributable risk

-1 to +1 -1 = risk greater in non-exposed group

Range of values of relative risk

0 to infinity

Meaning of value of relative risk

<1 = risk in exposed is less than risk in non-exposed 1 = risks are equal in both groups

Range of values for odds ratio

0 to infinity