Measures of Occurrence

Question 1

Incidence Proportion (R): What is the definition?

Answer 1

This is the proportion of people at risk for a particular disease (or outcome of interest) who develop that disease by end of specific time period of observation.

Question 2

How do you calculate R?

Answer 2

of persons developing disease during time period/# of persons at risk followed for that time period

Question 3

Define “risk”

Answer 3

The probability that an individual develops disease (or outcome) over a specified time period.

The realized probability for an individual is (usually) 0 or 1.

Question 4

Define “Incidence Proportion”

Answer 4

This is the average risk of disease (or outcome) in a group of individuals over time (t)
Summarizes all the 0’s and 1’s

Question 5

Difficulties in est. IP/Risk:
1. Dynamic population
2. Loss to follow up
3. Competing risks

Answer 5

1. The people at the beginning of the study may not be the same people at the end of the study
2. people die, quit
3. Example of competing risks: 20-year risk of Parkinson’s disease among older smokers compared with corresponding risk among older non-smokers

Question 6

Incidence Rate (I)

Answer 6

The rate, or velocity, at which new cases of a particular disease (or outcome of interest) occur in a population at risk for disease

Question 7

Incidence rate calculation

Answer 7

of persons developing a disease/total time at risk experienced by persons followed [“person-time”]

Question 8

What is the relationship between Incidence Proportion (Risk) and Incidence Rate?

Answer 8

If incidence rate is constant
Risk = 1-exp(-IRxT) = 1-e^(-1xtime)

Question 9

Rounding to significant figures for this class

Answer 9

2 significant figures: 20, 2.1, 0.00046
3 significant figures: 3.14, 303, 0.103

Question 10

What are the measures of disease frequency?

Answer 10

Incidence proportion
Incidence rate
prevalence proportion

Question 11

What can the mathematical relationship between prevalence, incidence rate and disease duration be used for?

Answer 11

to obtain estimates of prevalence and disease duration

Question 12

Example of incidence Proportion

Answer 12

Incidence = (New Cases) / (Population x Timeframe)

An example will help demonstrate this equation and is provided below.

You watch a group of the 5,000 people in your town. During a five-year period, 25 individuals are newly diagnosed with diabetes mellitus. What is the annual incidence of diabetes mellitus for your town?

(25 new cases diabetes mellitus)/(5,000 people x 5 years) =
(25 new cases) / (25,000 people-year) =
0.001 cases/people-year =
1 case / 1000 people-year