S2) Sampling Variation, Bias & Confounding

Question 1

Differentiate between statistics and probability, using a scenario

Answer 1

• Statistics: Given the information in your hand, what is in the pail?
• Probability: Given the information in the pail, what is in your hand?

Question 2

What is sampling distribution?

Answer 2

Sampling distribution is distribution of a statistic across an infinite number of samples

Question 3

Discuss how we analyse the following:

• Theoretical population
• The study population
• The sampling frame
• The sample

Answer 3

- Theoretical population: Who do you want to generalise to?

- The study population: Which population can you access?

- The sampling frame: How can you get access to them?

- The sample: Who is in your study?

Question 4

What are the three main studies in population science?

Answer 4

• Survey
• Cohort study
• Case-Control study

Question 5

In terms of the following image, describe the bias and precision.

Answer 5

• Large bias
• High precision

Question 6

In terms of the following image, describe the bias and precision:

Answer 6

• Large bias
• Low precision

Question 7

In terms of the following image, describe the bias and precision:

Answer 7

• No bias
• Low precision

Question 8

In terms of the following image, describe the bias and precision:

Answer 8

• No bias
• High precision

Question 9

What is a confidence interval?

Answer 9

Confidence interval is an estimate of the precision of the observed values in the sample

Question 10

What is a 95% confidence interval?

Answer 10

95% Confidence interval is the range that is likely to contain the mean of the population 95% of the time

Question 11

Which two factors widen the 95% confidence interval?

Answer 11

• Greater variation in population values
• Smaller sample sizes used to calculate it

Question 12

What approximates to the mean of population values?

Answer 12

The mean of the ‘mean of samples’ approximates to the mean of population values

Question 13

What approximates to normal Gaussian distribution?

Answer 13

The distribution of the mean of samples approximates to normal Gaussian distribution

Question 14

Which kinds of errors result from selection bias?

Answer 14

• Errors in generalisation
• Errors in comparability

Question 15

What are errors in generalisation?

Answer 15

Errors in generalisation: study participants are drawn from a sampling frame that is not representative of the general population

Question 16

What kinds of errors in comparability occur?

Answer 16

Errors in comparability:

• Groups beings compared are not from the same population
• One of the groups being studied is not representative of the sampling frame from which it was drawn

Question 17

Differentiate between direct and indirect standardisation

Answer 17

• Direct standardisation: directly standardised rates
• Indirect standardisation: standardised mortality rates

Question 18

What sorts of error lead to information bias?

Answer 18

• Differential recall error
• Differential observer error
• Differential measurement error
• Differential mis-classification error

Question 19

What is a confounding factor?

Answer 19

Confounding factor is an unobserved exposure associated with the exposure of interest and is a potential cause of the outcome of interest

NB. CONFOUNDING IS NOT ERROR