Lecture 5 - Measuring disease occurrence

Question 1

What is exposure and what is outcome?

What about population?

Answer 1

Exposure: possible determinant of health e.g. genetics

Outcome: health status

Population: group of ppl w/ shared characteristics (not necessarily the whole NZ etc).

• It forms the basis/denominator. Doesn’t include everyone in that area.
• Sometimes only need population at risk (e.g. women can’t get testicular cancer so they aren’t in the population at risk)

Question 2

What are the three main measures of outcome?

Answer 2

1. Prevalence: proportion of a population who have the exposure/outcome at a point in time, “point prevalence” (basically a snap shot)
2. Cumulative incidence
3. Incidence Rate

Question 3

Prevalence

1. Formula
2. Reporting it
3. Limitations of it

Answer 3

1. Existing cases/population at time t (the existing cases are always a subset of population)
2. Measure of occurrence……exposure/outcome…..population….time point…..value
   e. g. the prevalence of performance enhancing drug use in 2016 PUBH students during the 2016 PUBH final exam was 5.2%
3. Partly measures disease duration

Question 4

Incidence

1. What is it?
2. What’re the two types?

Answer 4

1. The occurrence of new cases of an outcome in a population during a specific period of follow-up (following up people to see if they develop the outcome)
2. Cumulative and incidence rate (different because of the denominator used)

Question 5

Cumulative incidence

1. What is it?
2. Formula?
3. Reporting it
4. What does it tell us?
5. What are its limitations

Answer 5

1. The proportion of an outcome-free population that develops the outcome of interest in a specified time period
2. (New cases during time period)/(population at risk in the beginning) during time period
3. Measure of occurrence……exposure/outcome…..population….time point…..value
   e. g. the cumulative incidence of death in legoville residents for the year 2015 was 1.1% (usually turn occurrence into %)
4. It tells us risk (average) of developing it (not individual risk; group risk)
5. It assumes closed population (no migration) and it’s highly dependent on follow-up time period (increase time = increase CI) because denom stays same regardless of time but numerator keeps increases so it’s important to state what the follow up time period is

Question 6

Incidence rate

1. What’s the denominator?
2. When do you stop being at risk?
3. Formula
4. Reporting
5. What does to tell you?
6. Limitations

Answer 6

1. Person-Time at risk as denom = sum of everyone in the population’s time at-risk of becoming case
2. When you become a case, are lost to follow up (die, move away) or follow up time ends
3. IR = (no. of new cases during period t/total person-time at risk of becoming case during t)
4. Measure of occurrence……exposure/outcome…..population………value
   e. g. the incidence rate of cancer in legoville residents was 50 per 100 person-years (don’t needa put time because it’s already part of it)
5. The rate at which new cases of the outcome of interest occur in a population
6. Complex to calculate and person-time not available

Question 7

Why is incidence useful?

Answer 7

Develop vs have
Not dependent on disease duration - useful when looking at increase or decrease risk of getting a disease (i.e. developing it)

Question 8

What is the relationship between prevalence and incidence? What does it give you?

Answer 8

Duration

P = I x D

This gives us insight into implications of altering incidence or disease duration

Question 9

Comparing measures of occurrence across populations: Age standardisation

1. Is the risk ____-____?
2. So how to solve the problem?
3. When do we do this?

Answer 9

1. Age-related
2. Age standardise
3. When age structures are different or when disease risk varies by age