Disease incidence and prediction.

Question 1

Tests that define an exposure or a disease status are subjected to what sort of error?

Answer 1

Systematic and random error.

Question 2

Is random or systematic error important in regards to bias?

Answer 2

Systematic.

Question 3

What two things can result in information bias?

Answer 3

1. Imperfect definitions of study variables.

2. Flawed data collection procedures.

Question 4

What can information bias result in?

Answer 4

Misclassification of exposure or disease.

Question 5

What two types of misclassification bias exist?

Answer 5

1. Non differential.

2. Differential.

Question 6

What sort of bias exhibits bias towards the null (in terms of the Odds Ratio)?

Answer 6

Non differential.

Question 7

Misclassification bias is similar but different to information bias. True or false?

Answer 7

False, misclassification bias is a type of information bias.

Question 8

Is the OR higher, lower, the same, or both in differential misclassification bias?

Answer 8

Both, it can be higher or lower.

Question 9

Define non differential misclassification bias.

Answer 9

The same amount of bias towards exposed/ non exposed state is the same in both the cases and control group.

Question 10

Define differential misclassification bias.

Answer 10

The same amount of bias towards exposed/ non exposed state is different in the cases and control group.

Question 11

What defines the ‘True status’ of a disease when testing a new diagnostic test?

Answer 11

The current gold standard test.

Question 12

Define sensitivity.

Answer 12

Proportion of individuals with a disease which test positive.

Question 13

Define specificity.

Answer 13

Proportion of those without a disease who tested negative.

Question 14

What is the equation for sensitivity?

Answer 14

TP/(TP+FN)

Question 15

What is the equation for specificty?

Answer 15

TN/(TN+FP)

Question 16

What is the equation for false positivity rate?

Answer 16

FP/(FP+TN) –> FP/ all non diseased

Question 17

What is the equation for false negative rate?

Answer 17

FN/(FN+TP) -> FN/ all diseased

Question 18

Define ‘Positive Predictive Value’ (PPV).

Answer 18

Proportion of positive tests that correctly identify diseased individuals.

Question 19

Define ‘Negative Predictive Value’ (NPV)

Answer 19

Proportion of negative tests that correctly identify non-diseased individuals.

Question 20

What is the equation for PPV?

Answer 20

TP/(TP+FP).

Question 21

What is the equation for NPV?

Answer 21

TN/(TN+FN)

Question 22

Which of the following vary with as disease prevalence varies?

1. Specificity.
2. Sensitivity.
3. PPV.
4. NPV.

Answer 22

PPV/NPV.

Question 23

Despite the fact that PPV and NPV are influenced by prevalence a good test will always show a good PPV. True or false?

Answer 23

False. A good test can result in a poor PPV if the prevalence is low.

Question 24

Is PPV or NPV more commonly used?

Answer 24

PPV (specificity is used more than sensitivity also).

Question 25

Disease prevalence does not modify sensitivity or specificity. What does?

Answer 25

The chosen cut of value of a continuous variable.

Question 26

What does lowering the cut of point of a test do? Why does it do this?

Answer 26

Improves sensitivity but lowers specificity. More false positives.

Question 27

What does increasing the cut of point of a test do? Why does it do this?

Answer 27

Lowered sensitivity but improved specificity. More false negatives.

Question 28

When is cut point decision in disease status especially important?

Answer 28

When the distribution of characteristic is 'unimodal'. This is because the 'grey area' is so large.

Question 29

Is a test with a sensitivity of 50%/ specificity of 50% better or worse than a test of sensitivity of 1%/ specificity of 1% better?

Answer 29

Sensitivity of 1%/ specificity of 1%. This is because if you swap the definitions over it becomes 99%/ 99%.

Question 30

What does ROC stand for and why is it used?

Answer 30

Receiver Operator Characteristics. Used to analyse the trade of between sensitivity and specificity.

Question 31

Would a ROC curve closer to the top left or bottom left corner represent a better test?

Answer 31

Top left corner.

Question 32

What is the AUC for a ROC curve and what does it show?

Answer 32

Area under the curve. When this is 1 the test is perfect.

Question 33

What does a diagonal line of 45 degrees on a ROC curve represent?

Answer 33

A test with no diagnostic value (same amount of true positives and false negatives).

Question 34

What can you correct for if sensitivity and specificity are known?

Answer 34

Misclassification, providing you have the data for correctly classified subjects.

Question 35

Technological advances have allowed for mass screening. Name one of these advances.

Answer 35

Genomics.

Question 36

What two types of screening are there?

Answer 36

1. Population based screening. | 2. Case finding.

Question 37

What is the pre clinical phase (PCP) between?

Answer 37

The phase between development of the pre clinical marker and normal clinical presentation.

Question 38

What does the prevalence of PCP correlate to?

Answer 38

How much earlier disease there is to detect?

Question 39

How often does screening for colorectal cancer occur?

Answer 39

Every 7-10 years.

Question 40

What is lead time?

Answer 40

The time by which a diagnosis was made earlier by screening.

Question 41

Describe lead time bias.

Answer 41

Screened and not screened cases may have equal survival times, but screened individuals look like they've survived longer as they've survived longer.

Question 42

Describe length time bias.

Answer 42

Screening can identify slow growing/less progressive cases which have better prognosis resulting in better survival anyway.

Question 43

Describe compliance bias/selection bias.

Answer 43

Volunteers are often better educated and healthier than normal individuals meaning they would have a better prognosis anyway.

Question 44

What 5 factors effect the feasibility of screening?

Answer 44

1. Burden of disease/ treatment effectiveness without screening. 2. Acceptability. 3. Efficacy (how well can it reduce mortality). 4. Efficency 5. Risk/ benefit balance.

Question 45

What 4 things effect the acceptability of screening?

Answer 45

1. Convenience. 2. Comfort. 3. Safety. 4. Costs