# Measures of Association Flashcards

1
Q

R = is defined as?

A

risk/prevalence

2
Q

E+ = is defined as?

A

exposed positive

3
Q

What does this equation tell you?

A

How many individuals out of the total have the disease?

4
Q

E-

A

exposed negative

5
Q

What does this equation tell you?

A

How many individuals out of the total do not have the disease?

6
Q
A

Often reflects study population, not general population?

7
Q

Example: Cohort study investigating feeding cats dry food (DF) and feline lower urinary tract disease (FLUTD). Based on the information in the table below, calculate the RE+, RE-, and Rtotal.

A

B/c we are looking at NEW cases of disease, it is incidence risk.

8
Q

What is risk ratio (RR) also known as?

A

Relative Risk, Incidence Risk Ratio

9
Q

How do you calculate the Risk Ratio (RR)?

A

Risk of exposed vs risk non-exposed

10
Q

Calculate the RR for the FLUTD study we discussed above. Here are the values again:

A

4 x more risk for cats that ate the cat food versus those that did not eat the dry food. Can also be said “The risk of FLUTD is 4 times higher for cats on DF.”

11
Q

How do you calculate Odds ratio?

A
12
Q

What is the purpose of an odds ratio?

A

Odds: we are comparing in the exposed group looking at those that have the disease compared to those that do not have the disease, ALL int he exposed group.

13
Q

What is this formula telling you?

A

The odds of disease in my exposed group divided by the odds of diseased in my non-exposed group.
Used in cohort and cross-sectional studies

14
Q

It does not matter if we are looking at the odds of exposure in an exposed or diseased group. The difference is that some study designs start by first grabbing cases, sampling from population to grab controls. But in this scenario, when speaking about prevalence/risk makes no sense b/c you are dictating how many cases are in your study so you can not talk about risk in a case control study. You can speak about odds b/c in a case control study, we are interested in the odds of exposure in disease group and then look at healthy animals and look at odds of exposure in them. It does not matter which order you do it b/c you always get the same answer.

A
15
Q

What is this formula telling you?

A

What are the odds of disease in my disease group divided by the odds of disease in my non-diseased group.
Used in case control-studies.

16
Q

Example: Cohort study investigating feeding cats dry food (DF) and feline
lower urinary tract disease (FLUTD).
Calculate the OE+, OE-, and OR.

A
17
Q

What is the effect in this case?

A

No effect
Risk and/or odds of disease are the same.
When there is no difference between exposed and non-exposed, you will get a 1.

18
Q

What is the effect in this case?

A

Increased risk

19
Q

What is the effect in this case?

A

Reduced risk (Protective)

20
Q

If disease is rare in underlying population
e.g. prevalence <5%, how does this effect the OR in relation to RR?

A

OR = RR

21
Q

If RR is close to null (RR=1), how does this effect the OR in relation to RR?

A

OR≈RR

22
Q

Track protection and risk on this line

A

Odds ratio, mathematically speaking, will always be greater than the risk ratio. They are almost the same when they are close to one.

23
Q

Measures of effect are usually expressed in?

A

‘absolute’ effect measure
* Related more to a number of cases an exposure caused
* Rather than relative ‘strength’ = ratio

24
Q

There is always an assumption of causal relationship. Why?

A

“Effect” will be the result of exposure IF the association is causal
* All interpretations in this presentation assume causality!

25
Q

If you were to remove that exposure, how much less disease would I expect to have in the animals.

A

Computed for:
* Exposed group
* Population

26
Q

Risk difference

A

It is defined as how much difference is the risk in one group vs another group.

27
Q

What is the level in the risk of the exposed compared to the risk in nonexposed on non? terms?
Just wanted to know How much reduction do you see in risk? How do you calculate this?

A
28
Q

Calculate the RD based on the information below:

A
29
Q

Attributable fraction (AF) is defined as?

A

AF is not a reduction completely, which is a risk difference, we want to know what the reduction is in proportion to all exposed. Why? Unclear
Fraction is key here.

You want to know what is the fraction of all the exposed animals that is attributed to the risk factor.

30
Q

Calculate the Attributable fraction based on the information below

A
31
Q

In vaccine trials, for example, when you have the vaccine it is a risk factor and it is going to prevent you from getting disease. It is very common in vaccine trials to report is as a preventable fraction. It is identical in way of thinking to attributable fraction.

A
32
Q

Calculate the preventable fraction.

A
33
Q

AF or PrevF is always between ___ and ___

A

0, 1

34
Q

What are population-level estimates?

A

If we remove exposure from population completely, how many lives are saved? Or how much risk or disease are we going to create at the population level?
CAUTION: Making assumption that you know the level of exposure in a population. These are all artificial numbers, doesn’t necessarily mean it’ll work at population level.
Extrapolate results to whole population
* Instead of just measuring effect on the exposure group
* Useful for population-level recommendations/policy making
* Only appropriate if source population exposure information is available
* Proportion of E+ is the same in the study as in the population

If exposure is rare, applying mitigation strategies will have a very small impact on the population

35
Q

If exposure is rare:
* Applying mitigation strategies will have a very small impact on the population

A
36
Q

Define population attributable risk (PAR).

A

Looking at difference in risk in the overall population compared to those that were exposed?

37
Q

Calculate the PAR for the information given:

A
38
Q

What should you avoid expressing PAR as?

A
39
Q

Calculate the PAF for the following information:

A
40
Q

How does PAF become a PPrevF?

A
41
Q

Is the population level estimate useful in the early stages of rolling out vaccines?

A

No, b/c few people in the population have been vaccinated

42
Q

Practice questions:

A
43
Q
A