BS - Epi/Biostat (Incidence vs. Prevalence, Risk, Precision v. Accuracy)

Question 1

Give the equation for incidence rate. What does it look at?

Answer 1

Incidence rate = # of new cases in a specified time period / Population at risk during same time period; Incidence looks at new cases (Think: “Incidents”)

Question 2

Give the equation for prevalence. What does it look at?

Answer 2

Prevalence = # of existing cases / Population at risk; Prevalence looks at all current cases; Think: “prevALence & ALL current cases”

Question 3

What equation approximates the relationship between prevalence and incidence rate?

Answer 3

Prevalence ~ incidence rate x average disease duration

Question 4

What is the incidence and prevalence for the common cold, and why?

Answer 4

Incidence and prevalence for common cold are very similar since disease duration is short

Question 5

What is the incidence and prevalence like for chronic diseases? Give an example of a chronic diseases.

Answer 5

Prevalence > incidence for chronic diseases (e.g., diabetes)

Question 6

In what type of study is odds ratio (OR) typically used? What defines it? What is its equation?

Answer 6

Typically used in case-control studies. Odds that the group with the disease (cases) was exposed to a risk factor (a/c) divided by the odds that the group without the disease (controls) was exposed (b/d). OR = (a/c) / (b/d) = ad / bc

Question 7

In what type of study is relative risk (RR) typically used? What defines it? What is its equation?

Answer 7

Typically used in cohort studies. Risk of developing disease in the exposed group divided by risk in the unexposed group (e.g., if 21% of smokers develop lung cancer vs. 1% of nonsmokers, RR = 21 / 1 = 21). RR = [a/(a+b)] / [c/(c+d)]

Question 8

In what context are RR and OR essentially the same?

Answer 8

If prevalence is low, RR ~ OR

Question 9

What defines Relative risk reduction (RRR)? What is its definition?

Answer 9

The proportion of risk reduction attributable to the intervention as compared to a control. RRR = 1 - RR (e.g., if 2% of patients who receive a flu shot develop flu, while 8% of unvaccinated patients develop the flu, then RR = 2/8 = 0.25, and RRR = 1 - RR = 0.75)

Question 10

What defines Attributable risk (AR)? What is its equation?

Answer 10

The difference in risk between exposed and unexposed groups, or the proportion of disease occurrences that are attributable to the exposure (e.g., if risk of lung cancer in smokers in 21% and risk in nonsmokers is 1%, then 20% (or .20) of the 21% risk of lung cancer in smokers is attributable to smoking). AR = (a / a + b) - (c / c + d)

Question 11

What defines Attributable risk reduction (ARR)?

Answer 11

The difference in risk (not the proportion) attributable to the intervention as compared to a control (e.g., if 8% of people who receive a placebo vaccine develop flu vs. 2% of people who receive a flu vaccine, then ARR = 8% - 2% = 6% = .06)

Question 12

Draw the chart that relates disease, risk factors, and the a-d letters.

Answer 12

See p. 53 in First Aid 2014 for visual near bottom of page

Question 13

What defines the number needed to treat? How is it calculated?

Answer 13

Number of patients who need to be treated for 1 patient to benefit. Calculated as 1/ARR.

Question 14

What defines the number needed to harm? How is it calculated?

Answer 14

Number of patients who need to be exposed to a risk factor for 1 patient to be harmed. Calculated as 1/AR.

Question 15

What defines precision?

Answer 15

The consistency and reproducibility of a test (reliability). The absence of random variation in a test.

Question 16

What reduces precision in a test? What does increased precision decrease?

Answer 16

Random error - reduces precision in a test. Increased precision => decreased standard deviation.

Question 17

What defines accuracy?

Answer 17

Trueness of test measurements (validity). The absence of systematic error or bias in a test.

Question 18

What reduces accuracy in a test?

Answer 18

Systematic error - reduces accuracy in a test