Statistics

Question 1

How do you calculate the positive likelihood ratio?

Answer 1

Positive likelihood ratio = sensitivity / (1 - specificity)

Question 2

Describe what percentage of people like within 1, 2, and 3 standard deviations from the mean

Answer 2

• 1 standard deviation account for 68% of the values
• 2 standard deviations account for 95% of the values
• 3 standard deviations account for 99.7% of the values

Question 3

Describe the positive predictive value and how to calculate it

Answer 3

• If the test is positive, what is the chance the patient has the disease?
• PPV = true +ve/true+ve and false +ves

Question 4

Describe the negative predictive value and how to calculate it

Answer 4

• If the test is negative, what is the chance the patient doesn’t have the disease?
• NPV = true -ve/true-ve and false -ves

Question 5

Describe sensitivity and how to calculate it

Answer 5

• True positive rate
• Sensitivity = true +ve/true +ve and false -ves
• A negative result in a test with a high sensitivity is useful for ruling out a disease

Question 6

Describe specificity and how to calculate it

Answer 6

• True negative rate
• Specificity = true -ve/true -ve and false +ves
• A positive result in a test with a high specificity is useful for ruling in a disease

Question 7

What is the value of a likelihood ratio?

Answer 7

• Assess the value of performing a diagnostic test
• Uses sensitivity and specificity of the test to determine whether a test result usefully changes the probability that a condition exists
• If you have a pre-test probability and a likelihood ratio, you can work out the post-test probability (+ve LR increases this, -ve LR decreases this)

Question 8

Describe the positive likelihood ratio and how to calculate it

Answer 8

• Likelihood ratio of a positive test is the probability of a true positive to a false positive
• +ve LR = sensitivity/100% - specificity

Question 9

Describe the negative likelihood ratio and how to calculate it

Answer 9

• Likelihood ratio of a negative test is the probability of a false negative to a true negative
• -ve LR = 100% - specificity/ sensitivity

Question 10

What is the difference between incidence and prevalence?

Answer 10

• Incidence is a rate e.g. cases per 5 years

- Prevalence is a cross section in time e.g. point prevalence studies

Question 11

What is the most important property of a screening test?

Answer 11

Sensitivity

Question 12

What are the criteria for a screening test (Wilson’s criteria)?

Answer 12

1. The condition sought should be an important health problem.
2. There should be an accepted treatment for patients with recognized disease.
3. Facilities for diagnosis and treatment should be available.
4. There should be a recognizable latent or early symptomatic stage.
5. There should be a suitable test or examination.
6. The test should be acceptable to the population.
7. The natural history of the condition, including development from latent to declared disease, should be adequately understood.
8. There should be an agreed policy on whom to treat as patients.
9. The cost of case-finding (including diagnosis and treatment of patients diagnosed) should be economically balanced in relation to possible expenditure on medical care as a whole.
10. Case-finding should be a continuing process and not a “once and for all” project.

Question 13

What is the relative risk and how do you calculate it?

Answer 13

• The probability of an event occurring in the exposed group vs. the non exposed group (event occurring/ total events)
• RR = a/ (a+b)
  ————
  c/ (c+d)

Question 14

What is the odds ratio and how do you calculate it?

Answer 14

• The event occurring: event not occurring
• e.g. OR of a baby girl is 1:1, probability (RR) is 50%
• Used when population risk is unknown in case-control studies. With low prevalence, OR~RR
• OR = ad/cb

Question 15

What is the absolute risk and how do you calculate it?

Answer 15

• The real difference in absolute terms between the two exposure or treatment groups
• Same as the risk difference
• e.g. if death occurs in 20% of placebo and 10% of drug, then RRR = 50%, but ARR = 10%

Question 16

What is the number needed to treat and how do you calculate it?

Answer 16

• The number of people needed to be treated to avoid one adverse outcome
• NNT= 1/ (incidence of unexposed – incidence of exposed)
• NNT = 100%/ARR = 1/ (probability in exposed - probability in non-exposed)

Question 17

What is the number needed to harm and how do you calculate it?

Answer 17

• NNH = 100%/ARI (absolute risk increase)

Question 18

What is a type 1 error?

Answer 18

= alpha error = false positive = p value

• Rejecting a null hypothesis when it is actually true
• Usually 0.05

Question 19

What is a type 2 error?

Answer 19

= beta error = false negative

- failing to reject the null hypothesis when it should have been rejected

Question 20

What increases the risk of a type 2 error (rejecting a good treatment)?

Answer 20

= beta error = false negative
= failing to reject the null hypothesis when it should have been rejected
= risk when the study is underpowered

Question 21

What is the power of a study and what is it determined by?

Answer 21

• The power is the probability that the test will reject a false null hypothesis
• Power = 1 - beta error (false negatives) = sensitivity
• Usually 0.8 or 0.9
• Determined by: population size, size of effect, variance within populations

Question 22

Describe the phases of drug design

Answer 22

Stage 0 - first in human studies, looks at pharmacokinetics and pharmacodynamics in humans, single sub-therapeutic dose given
Stage 1 - safety, healthy volunteers, determines safe dosing ranges, identify adverse events
Stage 2 - safety and efficacy (placebo), dosing requirements. Some combine stage 1 + 2
Stage 3 - confirmation of safety and efficacy, compare to active treatment (assess gold standard), RCT, requires approval from FDA
Stage 4 - continual pharmacovigilance (post marketing surveillance), new uses/populations, children, larger populations but less controlled, longer follow-up, interactions with other meds

Question 23

What are types of observational studies?

Answer 23

• Cohort - divide on exposure yes or no, prospective or retrospective
• Case-control - divide on disease yes or no
• Cross-sectional e.g. community survey
• Longitudinal - follow over time
• Ecological - 1+ variable measured at a group level

Question 24

What is the difference between internal and external validity?

Answer 24

• Internal - study well designed and minimises possibility of systematic bias (randomisation, blinding, ITT, published protocol, allocation concealment)
• External - applicable to a different unit/environment (multi-center, wide inclusion, limited exclusion criteria)

Question 25

What is random bias?

Answer 25

- Results due to sampling variability or measurement precision - Can be minimised but not voided

Question 26

What is systematic bias and what are examples?

Answer 26

- Reproducible inaccuracies that produce consistently false patterns of differences between observed and true values - Can be avoided by a good design - Selection bias, performance bias, detection bias, recall bias, attrition bias, reporting bias

Question 27

What is attrition bias?

Answer 27

Differences in withdrawal rates (incomplete outcome data)

Question 28

What is response bias?

Answer 28

The participant consciously or unconsciously gives a response that they think the interviewer wants to hear.

Question 29

What is optimistic bias?

Answer 29

Optimistic bias refers to overestimating the probability of positive events and underestimating the probability of negative events happening to themselves or their own children in the future, while they may be more objective in estimating outcomes for other children

Question 30

What is confirmation bias?

Answer 30

During the interpretation of data, the researcher looks for evidence to support their pre-existing beliefs.

Question 31

What is recall bias?

Answer 31

The outcomes of the treatment (good or bad) influence participants’ recollections of events prior to or during the treatment process.

Question 32

What is the best measure to describe the frequency of occurrence in an epidemic?

Answer 32

- Prevalence is the number of new and existing cases of a disease in a population at a specific point in time - The best measure to determine an epidemic is prevalence, because an epidemic refers to an increase in the number of cases of disease above what is normally expected in a specified population and region at a specific time.

Question 33

What is a student's t-test?

Answer 33

- Compares two samples of data | - Tests probability that two normally-distributed samples come from a population with the same mean value

Question 34

What is a chi square test?

Answer 34

Chi-square test evaluates if there is a significant difference between expected frequencies and observed frequencies in one or more sets of categorical data

Question 35

How do you calculate precision?

Answer 35

true positives/ (true positives + false positives) | = positive predictive value

Question 36

What is a Fisher's exact test?

Answer 36

Used to determine if there are non-random associations between two categorical variables