Epidemiology

Question 1

What is risk?

Answer 1

Probability an event will occur

Think about it in terms of prevalence and incidence

Question 2

What is prevalence

Answer 2

Proportion of sampled individual affected AT A GIVEN POINT IN TIME.

Includes both old and new cases, and is typically expressed as a percent.

Question 3

What is incidence?

Answer 3

Number of new cases occurring in a specified population over a SPECIFIED PERIOD OF TIME.

Typically expressed as cases per individual per unit of time.

Question 4

What is the formula for prevalence using the 2x2 table?

Answer 4

Prevalence = (a+c) / (a+b+c+d)

All animals that actually have the disease / total population

Question 5

What is sensitivity?

Answer 5

TRUE POSITIVE RATE

Likelihood of a positive test in individuals known to have the disease based on the gold standard

Question 6

What is specificity?

Answer 6

TRUE NEGATIVE RATE

Likelihood of negative test in individuals known NOT to have the disease based on the gold standard

Question 7

Do sensitivity and specificity change with disease prevalence?

Answer 7

NO - they are inherent properties of the diagnostic tests and do not change with disease prevalence in the population in question

Predictive values will change with disease prevalence

Question 8

What is the formula for sensitivity using the 2x2 table?

Answer 8

Sensitivity = a / (a+c)

True positives / everyone that actually has the disease

Question 9

What is the formula for specificity using the 2x2 table?

Answer 9

Specificity = d / (b+d)

True negatives / everyone that doesn’t have the disease

Question 10

What is positive predictive value?

Answer 10

Likelihood of a positive test in an animal truly affected by the disease

Increases as prevalence of disease increases

Question 11

What is negative predictive value?

Answer 11

Likelihood of a negative test in an animal truly negative for the disease.

Decreases as prevalence of disease increases

A negative test result can be trusted less in a disease that is super common, but can be trusted more for a disease that is rare because it’s more likely to be true

Question 12

T/F: Positive and negative predictive value do not change with prevalence.

Answer 12

FALSE - they do. They are not determined by test characteristics, they are determined by prevalence of disease

Question 13

What is the formula for positive predictive value using the 2x2 table?

Answer 13

PPV = a / (a+b)

True positives / all test positives

Question 14

What is the formula for negative predictive value using the 2x2 table?

Answer 14

NPV = d / (c+d)

True negatives / all test negatives

Question 15

What is relative risk/risk ratio?

Answer 15

Ratio of risk in individuals exposed vs. not exposed to a risk factor

Relative risk = (risk in exposed individuals) / (risk in unexposed individuals)

Question 16

What is the formula for relative risk using the 2x2 table?

Answer 16

Relative risk = [a/(a+b)] / [c/(c+d)]

Question 17

RR = 1 means what?

Answer 17

There is no difference in the risk of a given outcome between individuals exposed and unexposed to the risk factor/intervention

Question 18

RR<1 means what

Answer 18

There is less risk of the outcome in individuals exposed to the risk factor/intervention than those unexposed.

THE INTERVENTION IS PROTECTIVE

Question 19

RR >1 means what

Answer 19

There is more risk of the outcome in individuals exposed to the risk factor/intervention

Question 20

What is an odds ratio?

Answer 20

Probability of an event happening vs. not happening

Question 21

When is odds ratio useful?

Answer 21

Useful to measure associations when incidence cannot be calculated (I.e. case-control studies)

Also useful when the frequency of outcome is low

Question 22

What is the formula for odds ratio using 2x2 table?

Answer 22

Odds ratio = (a x d) / (b x c)

probability that case was expose / probability that a control was exposed

Question 23

OR = 1 means what

Answer 23

NO association between the factor and outcome

Question 24

OR < 1 means what

Answer 24

NEGATIVE association between factor and outcome

Question 25

OR > 1 means what

Answer 25

POSITIVE association between factor and outcome