2- Sampling Variation, Bias and Confounding

Question 1

When sampling a population what do we need it to be?

Answer 1

• Unbias
• Representative
• Precise

Question 2

What are the two types of error that may occur in a study?

Answer 2

- Random (chance): decreases as sample size increases

- Bias (systematic): does not decrease. quantified by difference between true value and actually value

Question 3

How do you increase precision?

Answer 3

Increase sample size. Decreases uncertainty

Question 4

What are sources of bias?

Answer 4

- Selection Bias

- Information Bias

Question 5

What are some selection biases?

Answer 5

1. Non-representative study sample

2. Groups within study may not be comparable

3. Healthy worker effect (people who aren’t working may be due to illness and disease so not necessarily the fact they aren’t working that is causing them to die)

Question 6

What are some information biases?

Answer 6

- Recall error

- Interviewer/Observer error (predispositions)

- Measurement error (measurer or equipment)

• Misclassification (some people without disease diagnosed as having disease, mainly due to measurement or observational error)

Question 7

What does no bias, high precision look like?

Answer 7

Question 8

Does a large study mean less bias?

Answer 8

NO!

but means more precise

Question 9

Define prevalence.

Answer 9

The proportion of people who have a disease at a given point in time

NOT A RATE - per 1000 people then made into a percetage

Question 10

Define incidence and state how you can calculate incidence rate?

Answer 10

Incidence is the number of new cases of a diseases within a given time frame.

Useful when monitoring epidemics

Question 11

What is person years and how is it calculated?

Answer 11

Sum of the total time of everybody followed up in a study

e. g 3 people followed for 5 years and two people followed for 3 years =
(3x5) + (2x3) = 21 py’s

Question 12

How else can prevalence be measured?

Answer 12

Incidence x Duration of Disease

Question 13

What is the IRR and how is it calculated?

Answer 13

Incidence Rate Ratio

Used to compare rate of incidence in two groups

Question 14

What is relative risk? (a.k.a risk ratio)

Answer 14

Absolute risk of one group / Absolute risk of another group

The comparison of the probability of an event occuring in one group compared to another

Question 15

What factors affect the length of a disease?

Answer 15

• Mortality
• Curability

Question 16

How do you work out the odds of an event occuring?

Answer 16

It is a ratio

Diseased/Non-diseased for example

or

P/1-P

Question 17

What is the odds ratio?

Answer 17

Odds ratio is ratio of odds of outcome in groups defined by levels of exposure at a particular time

Comparison of odds of disease in two groups

Question 18

When can relative risk and odds ratios be used?

Answer 18

• When all patients in the groups have been consistently followed up

Question 19

What is the absolute risk of disease?

Answer 19

Disease/Diseased+Non-diseased

Question 20

How do we interpret OR?

Answer 20

- Ratio < 1: risk of the group of interest is less than the risk of the comparison group

- Ratio of 1: risk of the group of interest is the same as the risk of the comparison group

- Ratio > 1: risk of the group of interest is greater than the risk of the comparison group

Question 21

What is risk difference?

Answer 21

Absolution difference in risk between two groups. If zero there is no difference in risk

Question 22

What is a confounding variable?

Answer 22

An unobserved factor that impacts both exposure and can cause disease

or

When comparing groups the association between exposure and outcome is distorted by another variable

e.g alcohol, gender, mouth cancer

Question 23

How can we account for confounding variables?

Answer 23

- Standardisation: e.g weight, age, sex

• Some confounding variables unknown so difficult to address