Stats/Epidemiology

Question 1

What is prevalence?

Answer 1

1. The proportion of sampled individuals affected AT A GIVEN POINT IN TIME
2. Includes both old and new cases
3. Typically expressed as a percent

Question 2

What is incidence?

Answer 2

1. The number of NEW cases occurring in a specified population over a SPECIFIC PERIOD OF TIME
2. Typically expressed as cases per individual per unit of time

Question 3

What is the equation for prevalence in terms of a 2x2 table?

Answer 3

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

Prevalence = (animals with the disease)/(total population)

Question 4

What is sensitivity?

Answer 4

1. The likelihood of a POSITIVE test in individuals known to HAVE the disease
2. The TRUE POSITIVE rate

Question 5

What is specificity?

Answer 5

1. The likelihood of a NEGATIVE test in individuals that DO NOT HAVE the disease
2. The TRUE NEGATIVE rate

Question 6

What represents the TRUE POSITIVE rate?

Answer 6

Sensitivity

Question 7

What represents the TRUE NEGATIVE rate?

Answer 7

Specificity

Question 8

What is the equation for sensitivity in terms of a 2x2 table?

Answer 8

Sensitivity = a/(a+c)

Sensitivity = true positives/everyone that has the disease

Question 9

What is the equation for specificity in terms of a 2x2 table?

Answer 9

Specificity = d/(b+d)

Specificity = true negatives/everyone that does not have the disease

Question 10

What is the positive predictive value?

Answer 10

1. The likelihood of a POSITIVE test in an animal truly AFFECTED by the disease
2. INCREASES as PREVALENCE of disease INCREASES

Question 11

What is the effect on the positive predictive value as the prevalence increases?

Answer 11

The positive predictive value will increase

Question 12

What is the negative predictive value?

Answer 12

1. The likelihood of a NEGATIVE test in an animal truly NEGATIVE for the disease
2. Decreases as PREVALENCE of disease INCREASES

Question 13

What is the effect on the negative predictive value as prevalence increases?

Answer 13

The negative predictive value will decrease

Question 14

What is the equation for positive predictive value in terms of a 2x2 table

Answer 14

PPV = a/(a+b)

PPV = true positive/all animals testing positive

Question 15

What is the equation for negative predictive value in terms of a 2x2 table?

Answer 15

NPV = d/(c+d)

NPV = true negatives/all animals testing negative

Question 16

What is relative risk?

Answer 16

1. The ratio of risk in individuals exposed vs not exposed to the factor
2. Also called risk ratio

Question 17

What is the equation for relative risk in terms of a 2x2 table?

Answer 17

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

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

Question 18

Interpret:

Risk ratio = 1

Answer 18

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

Question 19

Interpret:

Risk ratio < 1

Answer 19

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

Question 20

Interpret:

Risk ratio > 1

Answer 20

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

Question 21

What is the odds ratio?

Answer 21

1. The probability of an event happening vs not happening

Question 22

When is it useful to use an odds ratio?

Answer 22

1. Useful to measure associations when INCIDENCE cannot be calculated (ie case control studies)
2. Useful when frequency of outcome is LOW (ie would take 80 years to get a large enough population)

Question 23

What is the equation for the odds ratio in terms of a 2x2 table?

Answer 23

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

Odds ratio = (probability that case was exposed)/(probability that a control was exposed)

Question 24

Interpret:

Odds ratio = 1

Answer 24

There is no association between the factor and the outcome

Question 25

Interpret: Odds ratio < 1

Answer 25

There is a negative association between the factor and the outcome

Question 26

Interpret: Odds ratio > 1

Answer 26

There is a positive association between the factor and the outcome

Question 27

What is the likelihood ratio?

Answer 27

use the sensitivity and specificity of the test to determine whether a test result usefully changes the probability that a condition (such as a disease state) exists

Question 28

What is the equation for the likelihood ratio in a positive test result?

Answer 28

Likelihood Ratio (+) = Sens/(1-spec)

Question 29

What is the equation for the likelihood ratio in a negative test result?

Answer 29

Likelihood Ratio (-) = (1-sens)/Spec

Question 30

Interpret: Likelihood Ratio = 1

Answer 30

The test is useless

Question 31

What would be a good likelihood ratio for a positive result?

Answer 31

LR (+) > 10

Question 32

What would be a good likelihood ratio for a negative result?

Answer 32

LR (-) < 0.1

Question 33

What is a type 1 error?

Answer 33

1. False positive rate (p-value) 2. Rejection of a true null-hypothesis 3. Ex: An innocent person is convicted

Question 34

What is a type 2 error?

Answer 34

1. False negative rate 2. Non-rejection of a false null hypothesis 3. Ex: A guilty person is not convicted

Question 35

With a cohort study, the (relative risk/odds ratio) should be used?

Answer 35

Relative risk

Question 36

With a case control study, the (relative risk/odds ratio) should be used

Answer 36

Odds ratio

Question 37

A cohort study can be (Prospective/Retrospective/Both)

Answer 37

Both

Question 38

A case control study can be (prospective/retrospective/both)

Answer 38

Retrospective

Question 39

What does incomplete penetrance mean in terms of inheritance patterns?

Answer 39

The person either expresses the disease phenotype or they do not when they have the disease

Question 40

Incomplete penetrance is associated only with (dominant/recessive) inheritance, never (dominant/recessive) inheritance

Answer 40

1. Dominant | 2. Recessive

Question 41

A high prevalence would be indicative of a(n) (acute/chronic) disease

Answer 41

Chronic

Question 42

If the incidence is higher than the prevalence, it would be indicative of a(n) (acute/chronic) disease

Answer 42

Acute

Question 43

Disinfectants that work in the presence of organic matter

Answer 43

1. Aldehydes 2. Phenols 3. Quaternary ammonium compounds (roccal)