POPH192- Lecture 16

Question 1

what do measures of occurence describe?

Answer 1

describes how often a disease pccurs in a population

Question 2

what are the 3 measures of occurence?

Answer 2

• prevalence
• incidence proportion
• incidence rate

Question 3

what is prevalence?

Answer 3

the proportion of a defined population who have a disease/outcome at a given point in time

Question 4

what is the equation for prevalence?

Answer 4

prevalence = people with disease at point in time / total population at point in time

Question 5

what is prevelance used to tell us?

Answer 5

to tell us how much disease/outcome is in a population and to help with resource allocation

Question 6

what is the generic interpretation for prevalence?

Answer 6

the prevalence of outcome** in **population** at **timepoint** was **XX.X%

Question 7

what are the limitations of prevalence?

Answer 7

• difficult to assess how disease/outcome developed
• influenced by duration of disease (diseases with longer duration have larger prevalence by default)

Question 8

what is incidence?

Answer 8

the occurence of new cases of an outcome in a population during a specific period of follow up

Question 9

what are the 2 types of incidence?

Answer 9

• incidence proportion
• incidence rate

Question 10

what is the incidence proportion equation?

Answer 10

IP = number of people who develop disease / number of people at risk at start of follow up period

Question 11

what is the incidence rate equation?

Answer 11

IR = number of people who develop disease / number of person years at risk

Question 12

which equation should you use if you are given number of person years at risk

Answer 12

use incident rate equation

Question 13

which equation should you use if you are given number of people at risk at start of follow up period?

Answer 13

incidence proportion equation

Question 14

what is the interpretation of incidence proportion?

Answer 14

the incidence proportion of outcome in population over time period was XX.X%

Question 15

what is the interpretation of incident rate?

Answer 15

the incidence rate of outcome in population was XX per X person years

Question 16

what are some important things to remember for incidence rate? (4)

Answer 16

• always convert to years
• x100 to convert answer to ‘per 100 person years’
• sometimes you may need to interpret person years at risk from a graph/table
• need to add up person years at risk of all participants in the figure

Question 17

when is a person so longer at risk?

Answer 17

when a participant drops out, dies, or develops the disease they are no longer at risk.

• only include ‘person years at risk’ in calculation
  e. g. a person dropped out after 8 years of a study, so they contributed 8 person years at risk

Question 18

what is the relationship between prevalence and incidence?

Answer 18

Prevalence is proportinal to incidence and duration

P ~ I x D

**do not use as an actual equation**

Question 19

why is age standardisation necessary?

Answer 19

populations are not all the same.
- to compare rates of disease, we need to standardise populations

Question 20

what is age standardisation?

Answer 20

a technique which allows populations to be compared when the age profiles of populations are different

Question 21

what are the 2 criteria for age standardisation?

Answer 21

1) the age structures of populations differ
2) the disease risk vary by age