Nested studies.

Question 1

What is the main advantage of using nested studies?

Answer 1

They are effective ways to utilise resources.

Question 2

What is an advantage of using a nested case control study over a normal case control study?

Answer 2

Exposure is assessed prior to the outcome.

Question 3

Nested case control studies reduce the wasting of resources. What is another advantage of them?

Answer 3

Reduce cost in data entry.

Question 4

Giving people the option to right written answers in questionaries can provide useful, detailed information. What is a disadvantage of these questions?

Answer 4

Free text fields are very difficult to code.

Question 5

Ideally controls would be selected from the study population. If this is not the case and an external group must be used what must you ensure is the case?

Answer 5

That the external group should have the same amount of exposure to the cases. This is rarely plausible.

Question 6

Should controls be selected independently or dependently of their exposure status?

Answer 6

Independently.

Question 7

What rate applied to controls should not differ dependant on exposure status?

Answer 7

The sampling rate.

Question 8

What three types of control sample strategies exist for nested controls?

Answer 8

1. Cumulative sampling.
2. Incidence density sampling.
3. Case cohort/ baseline.

Question 9

Which of the following are sampling methods used for nested studies?

1. Cumulative sampling.
2. Incidence density sampling.
3. Case cohort/ baseline.

Answer 9

1. Cumulative sampling.

2. Incidence density sampling.

Question 10

What is another name for incidence density sampling?

Answer 10

Risk set sampling.

Question 11

Describe cumulative sampling.

Answer 11

Cases are identified and controls are matched from survivors accordingly at the end of followup/ when you decide to do the study.

Question 12

Describe incidence density sampling.

Answer 12

Controls are selected at the same time that cases are diagnosed.

Question 13

Describe case cohort studies.

Answer 13

Non cases are a sub cohort of the original cohort.

Question 14

Which of the following sampling techniques can result in the same individual being classified as both a case and a control?

1. Cumulative sampling.
2. Incidence density sampling.
3. Case cohort.

Answer 14

Incidence density sampling.

Case cohort.

Question 15

What bias can be introduced in cumulative sampling and not in incidence density sampling?

Answer 15

‘Super healthy’ individuals have been selected from the ordinal cohort.

Question 16

Which of the following method of control selection is used for assessing multiple outcomes?

1. Cumulative sampling.
2. Incidence density sampling.
3. Case cohort.

Answer 16

Case cohort.

Question 17

Which of the following method of control selection allows cases to be matched to any number of controls?

1. Cumulative sampling.
2. Incidence density sampling.
3. Case cohort.

Answer 17

All of them.

Question 18

Control selection for case cohort studies should happen at the start of the study. True or false?

Answer 18

False, it can happen at any time. All individuals at the start of the study should however be considered for selection.

Question 19

In terms of matching for age what is an advantage of using a large cohort?

Answer 19

You have more chance of having sufficient data to make narrower age categories.

Question 20

What is the relationship between effect size and sample size?

Answer 20

The large the effect size the smaller the sample needed to detect an effect.

Question 21

Roughly how much cheaper is a nested case control study compared to analysing the whole cohort?

Answer 21

More than 99% cheaper.

Question 22

What is the issue of drawing blood samples for case control studies?

Answer 22

The blood sample may change after the development of a disease.

Question 23

What sort of ratio is calculated form cumulative density sampling?

Answer 23

Odds ratio (like normal case control).

Question 24

What sort of ratio is calculated form incidence density sampling?

Answer 24

Rate ratio.

Question 25

What sort of ratio is calculated form case cohort sampling?

Answer 25

Risk ratio.

Question 26

Case control studies are good for rare outcomes. Nested studies are not. True or false?

Answer 26

False, they are.

Question 27

Why are nested case-control studies flexible?

Answer 27

As you can do them at any time.

Question 28

Why does a nested case control study reduce bias?

Answer 28

Selection bias is reduced as the same population is used for cases and controls/ cases and non cases.

Question 29

Case cohort studies are good at excluding what sort of cases?

Answer 29

Cases that are logistically difficult to exclude as they have a large subclinical phase. To exclude an individual they need to be screened (which is easier when you are only using a subset for the control group).

Question 30

‘Case cohort studies are always ideal for testing multiple outcomes from a cohort as they are cheaper and waste less resources.’ Why is this statement false?

Answer 30

As excluding cases reduces statistical efficacy. If it is possible to use the whole cohort you should do so.