W1: Measures of Disease

Question 1

Considerations in measuring disease

Answer 1

1. The number of people affected
2. The size of the population
3. The length of time the population is followed

Question 2

Ratio

Answer 2

One number divided by another

Question 3

Rate

Answer 3

One number divided by another, with time as an integral part of the denominator

Question 4

Proportion

Answer 4

One number divided by another, and the two numbers are related (usually denominator includes the numerator)

Question 5

Incidence

Answer 5

Measures new occurrences of disease. Helpful in determining how quickly a disease is spreading. Most useful to evaluate whether a beneficial intervention prevents disease or a harmful exposure causes disease.

Question 6

Prevalence

Answer 6

Measures existing disease cases (which may include some new cases). Helpful in determining how common a disease is at a particular point in time in a population

Question 7

Cumulative incidence

Answer 7

Number of new cases / Number of people at risk
Fixed population
Assumes entire population has been followed the entire time
Dimensionless
Eg. 13% of women will develop breast CA at some point in their lives

Question 8

Incidence Rate

Answer 8

Number of new cases / Person-time at risk

Ranges from 0-infinity (0 means no new cases, infinity means all died instantaneously)

Question 9

Person-time

Answer 9

Starts when study starts following; stops when person gets the disease or is lost to followup/dies

Question 10

Point prevalence

Answer 10

Number of existing cases of disease at point in time / number in total population

Question 11

Period prevalence

Answer 11

Number of existing cases in period of time / Number in total population

Question 12

Risk difference

Answer 12

Additive measure of comparison. AKA excess risk, attributable risk. Non-causal interp: [%] is the excess risk of [disease] associated with [exposure]. Causal interp: [%] is the risk of [disease] that would be eliminated if [exposure] were eliminated.

Question 13

Population risk difference

Answer 13

Additive measure of comparison. AKA attributable risk. Risk in exposed - risk in unexposed. Non-causal interp: [%] is the excess risk of [disease] in the total population associated with [exposure]. Causal: [%] is the risk of [disease] that would be eliminated in the total population if [exposure] were eliminated.

Question 14

Attributable proportion among the exposed

Answer 14

Additive measure of comparison. AKA attributable risk. Gives what percentage of disease IN THE EXPOSED GROUP is attributable to the exposure (rest is background disease).

Question 15

Prevented fraction

Answer 15

Additive measure of comparison.

Question 16

Relative risk

Answer 16

Relative measure of comparison. AKA risk ratio, rate ratio, relative rate. Eg “the risk of [disease] is [x] times higher in the exposed group than in the unexposed.” Stays constant across all different levels of risk among the unexposed.

Question 17

Odds ratio

Answer 17

Relative measure of comparison. Same interpretation as for relative risk, but instead would be “x-fold increase in the ODDS”. As the risk among the unexposed changes, the numeric value of OR and RR get farther away from each other (OR increases as risk among unexposed increases)

Question 18

Attributable proportion in the total population

Answer 18

Additive measure of comparison. Gives what percentage of disease IN THE TOTAL POPULATION is attributable to the exposure (rest is background disease). Only has causal interpretation