Lecture 16: Measuring Disease Occurence

Question 1

Define epidemiology

Answer 1

The study of the occurrence and distribution of health-related events, states or processes in specified populations

Question 2

What are the two measures of occurrence of disease?

Answer 2

• prevalence

- incidence

Question 3

Incidence is one measure of occurrence. What are the two types of incidence?

Answer 3

• incidence proportion

- incidence rate

Question 4

Define prevalence

Answer 4

The proportion of a population who have the disease at a point in time

Question 5

Why do we care about the prevalence of disease?

Answer 5

Because it tells us about the burden of disease so we know where we can allocate resources

Question 6

How can you calculate prevalence?

Answer 6

the number of people with a disease at a given point in time divided by the total number of people in the population at that point in time

Question 7

What five things do we need to include when reporting prevalence?

Answer 7

1. measure of occurrence
2. exposure or outcome
3. population
4. time point
5. value

Question 8

How do you report prevalence of asthma among people taking POPH192 if the prevalence was 10% on the 10 Aug?

Answer 8

The prevalence of asthma in the POPH192 class on August 10th 2020 was 10%

Question 9

What are some limitations of using prevalence?

Answer 9

• it is difficult to measure the development of disease

- it is influenced by the duration of disease

Question 10

How does duration affect prevalence?

Answer 10

Because prevalence is looking at the number of people who have a disease in a given point in time, if the duration of the disease is longer, at one point in time, it may look like there are more people with the disease than if the duration was shorter

Question 11

Define incidence

Answer 11

the occurrence of new cases of an outcome in a population during a specified period of follow up

Question 12

Define incidene proportion

Answer 12

The proportion of an outcome-free population that develops the outcome of interest in a specified time period

Question 13

How do you calculate incidence proportion?

Answer 13

the number of people who develop the disease in a specified period divided by the number of people at risk of developing the disease at the start of the period

Question 14

Why might people not be considered “at risk” at the start of the study?

Answer 14

• they already have the condition

- the condition is something that they cannot develop

Question 15

What five things do we need to include when reporting incidence proportion?

Answer 15

1. measure of occurrence
2. exposure or outcome
3. population
4. time point
5. value

Question 16

How do you report incidence proportion of lower back pain among among nurses if the proportion was 35% in 12 months?

Answer 16

the incidence proportion of lower back pain in nurses in 12 months was 35%

Question 17

Why should we use the incidence proportion?

Answer 17

the incidence proportion tells us about the risk of developing an outcome

Question 18

What are two limitations of using incidence proportion?

Answer 18

• it assumes a closed population (does not account for people coming or going)
• it is highly dependent on the time period (longer time period means a higher incidence proportion)

Question 19

Define incidence rate

Answer 19

the rate at which new cases of the outcome of interest occur at a population

Question 20

How do you calculate the incidence rate?

Answer 20

the number people who develop the disease in a specified period divided by the number of person years at risk of developing the disease

Question 21

How do you calculate the number of person years at risk?

Answer 21

it is the sum of everyone in the population’s time at risk of becoming a case

Question 22

Why might some people stop being at risk when calculating person-years at risk?

Answer 22

• they become a case
• they are lost to follow up
• the follow up time ends

Question 23

What four things need to be included when reporting incidence rate?

Answer 23

1. measure of occurrence
2. outcome
3. population
4. value

Question 24

How do you report incidence rate of glandular fever in the class if the rate was 0.5 case per person year?

Answer 24

The incidence rate of glandular fever in the class was 50 per 100 person-years

Question 25

What are two limitations of the incidence rate?

Answer 25

1. there can be no person-time information available

2. it can complex to calculate

Question 26

How does prevalence relate to incidence and duration?

Answer 26

Prevalence approximates incidence multiplied by the disease duration (although this can’t be used as an actual formula) so changes to the incidence and duration can affect the prevalence

Question 27

When should we age standardise?

Answer 27

when the age structures between two populations differs and when the disease risk varies by age

Question 28

Why do we need to age standardise?

Answer 28

If the outcome of interest is an age related outcome and the age structures differ between the populations then if we don’t age standardise then the difference in occurrence we see could just be because of the difference in age