Week 11: Incidence Prevalence

Question 1

What the definition of incidence?

Answer 1

Incidence is a measure if the risk of developing some new condition within a period of time. Although sometimes loosely expressed simply as the number if new cases during some time period, it is better expressed as a proportion or rate

Question 2

Proportion: cumulative incidence

Answer 2

Incidence proportion: is the number of new cases within a specified time period divided by the size of the population initially at risk. Ie population contains 1000 non diseased, and 28 develop condition then invidence proportion is 28 cases per 1000 persons over a 2 yr period

Question 3

Incidence rate:

Answer 3

A measure of the frequency with which an event, such as a new case of illness, occurs in a population over a period of time .
Incidence rate per 100 000 =
New cases occurring during a given time/ population at risk during the same time period x 100 000

Question 4

Incidence rate:

The most appropriate denominator (on bottom) will be one of the following:

Answer 4

• average size if the population over the time period eg between jan- dec
• size if the population (either total or at risk) at the middle of the time period eg pop in June
• start of the population at the start of the time period eg pop in January

Question 5

Incidence expressed differently

Answer 5

The dominator as the sum if each persons time at risk. Why? For some this may be 1 year for others 15 months or 16 months etc. with simple arithmetic we can express thus as incidence ‘density’ as:
Number of new cases of a disorder during a given time period/ total person-time of observation
Eg 12.4 cases per 100 person years of observation

Question 6

Prevalence

Answer 6

Prevalence quantifies the proportion of individuals in a population who have the disorder at a specific point or period of time.
The formula for calculating prevalence is p:
Prevalence:
Number if existing cases if a disorder/ total population

Question 7

What are the two types of prevalence, explain them

Answer 7

Point prevalence: the proportion reflecting the number of existing cases of a disease at one time. Eg do you have back pain now
Period prevalence: the proportion reflecting the number if existing cases of a disease during an interval of time
Eg have you experienced back pain in the last 1 month