Lecture 27: Confounding |

Question 1

What is confounding?

Answer 1

A mixing or muddling of effects when the relationship we are interested in is confused by the effect of something else - the confounder

Question 2

What are the three properties of a potential confounder?

Answer 2

• independently associated with the outcome
• independently associated with the exposure
• not on the causal pathways

Question 3

What is meant by the confounder being independently associated with the outcome? Give an example

Answer 3

A potential confounder is a risk or protective factor for the outcome regardless of the exposure status.
For example in a study looking at the link between childhood antibiotics and obesity at age 5, SES is a potential confounder. By it being independently associated with the outcome, it means that SES is a risk factor for obesity (low SES means more likely to be obese) regardless of whether their child had antibiotics.

Question 4

What is meant by the confounder being independently associated with the exposure? Give an example

Answer 4

There needs to be different proportions of people with potential confounders across the exposure groups otherwise it won’t lead to a different outcome.
For example in a study looking at the link between childhood antibiotics and obesity at age 5, SES is a potential confounder. By it being independently associated with the exposure, it means that SES is a risk factor for childhood antibiotics (low SES means more likely to have antibiotics) regardless of whether their child is obese.

Question 5

What is meant by the confounder not being on the causal pathway? Give an example

Answer 5

The confounder is not the mechanism which the exposure affects the risk of the outcome.
For example in a study looking at the link between childhood antibiotics and obesity at age 5, SES is a potential confounder. By it being on the causal pathway, it means that whether a child has had antibiotics, it does not determine their SES and whether they are obese or not

Question 6

What four things can confounding do?

Answer 6

• can cause an overestimation of a true association
• can cause an underestimation of a true association
• can cause a change of direction of a true association
• can give an appearance of an association when there really isn’t one

Question 7

What three things can we do to control for confounding?

Answer 7

1. randomisation
2. restriction
3. matching

Question 8

Describe the process of randomisation

Answer 8

In an RCT, of the sample, people are randomly allocated into treatment group and control group

Question 9

For a harmful exposure, if the study RR is greater than the true RR, what has happened?

Answer 9

confounding has resulted in an overestimation of the true harmful effect of the exposure (the association appears stronger than it really is, the RR is further away from the null)

Question 10

For a harmful exposure, if the study RR is less than the true RR, what has happened?

Answer 10

Confounding has resulted in an underestimation of the true harmful effect of the exposure (the associated appears weaker than it really is, the RR is closer to the null)

Question 11

For a beneficial exposure, if the study RR is less than the true RR, what has happened?

Answer 11

confounding has resulted in an over-estimation of the true protective effect of the exposure (the association appears stronger than it really is, the RR is further from the null)

Question 12

For a beneficial exposure, if the study RR is greater than the true RR, what has happened?

Answer 12

Confounding has resulted in an underestimation of the true protective effect of the exposure (the association appears weaker than it really is, the RR is closer to the null)

Question 13

How can you identify potential confounders? Give an example

Answer 13

you have to plan ahead and collet information on all potential confounders and then you can look for an imbalance in potential confounders between groups eg. is there a big difference in age or SES between the two groups

Question 14

When can you control for confounding?

Answer 14

in the study design

in the analysis phase

Question 15

What are three ways to control for confounding in the study design?

Answer 15

• randomisation
• restriction
• matching

Question 16

What is the purpose of trying to control for potential confounders in the study design?

Answer 16

to try and make the groups being compared alike with regard to potential confounders

Question 17

What is randomisation? What is the advantage of of it and what are some potential issues?

Answer 17

• when people are randomly allocated into treatment or control groups
• the advantage to this is that is controls for potential confounders because if people are randomly allocated, there should be an even distributed of both known and unknown confounders between the two groups
• it works best with large sample size, it requires equipoise and an intention-to-treat analysis

Question 18

What is restriction? Give an example

Answer 18

when you restrict the sample to only one stratum of potential confounder eg. when looking at a study involving breast cancer, you may choose to exclude men

Question 19

What are some “buts” of restriction?

Answer 19

• it can reduce generalisability
• it reduces the number of potential participants
• there is the potential for residual confounding with imprecisely measured (or broadly defined) confounders
• you can usually only do it with one confounder

Question 20

What is matching and when is it done?

Answer 20

When you choose people for the comparison (control) group who have the same values of the potential confounding factor(s) as people in the exposed group (cohort studies) or case group (case-control studies)

Question 21

What are the two ways to do matching?

Answer 21

individual or frequency matching

Question 22

What is individual matching?

Answer 22

each case matched with one or more controls having the same confounding variable characteristics

Question 23

What is frequency matching?

Answer 23

when you match at an aggregated level

Question 24

What are some advantages and disadvantages of matching?

Answer 24

Positives:
- useful for difficult to measure/complex potential confounders
- can improve efficiency of case-control studies with small numbers
Negatives:
- individual matching can be difficult and limit the number of potential participants

Question 25

What are some disadvantages to controlling for confounding in the study design?

Answer 25

you can’t assess the association between the potential confounder and the outcome and you can’t assess whether the confounder is actually a confounder