Epidemiology Measuring Disease Frequency-Incidence

1. EPIDEMIOLOGY
Lecture 3:
Incidence

5. What is incidence?
• The rate at which events occur in a
population.
new

6. Numerator
Denominator
Multiplier
Number of NEW events in a specified period
X 10
n
The numerator strictly refers only to first events
of disease.

7. • Frequency Count
• Rate
• Proportion

8. • Average population exposed to risk during the
period.
– The average size of the population is often the
estimated population size at the midperiod. E.g.,
mid-year population when calculating annual
incidence rates.
The denominator of an incidence - 1

9. The denominator of an incidence - 2
• A person-time denominator (or other units
such as passenger-miles denominator).
– For each individual in the population, the time of
observation is the period that the person remains
disease-free.
– The denominator used for the calculation of
incidence is therefore the sum of all the disease-
free person-time periods during the period of
observation of the population at risk.

10. Example 1
0 1 1098765432
Duration (in years)
A
B
C
D
E
A
B
D
E
C
8 years
9 years
10 years
4 years
5 years
Total p-y = 36
Incidence= (4/36)x1000 = 111.11 per 1000 person-years
OR 111.11 per 1000 persons per year

12. Example 1
0 1 1098765432
Duration (in years)
A
B
C
D
E
A
B
D
E
C
5 years
6 years
10 years
6 years
1 year
Total p-y = 28
Incidence= (4/28)x1000 = 142.86 per 1000 person-years

13. Example 1
0 54321
Duration (in years)
A
B
C
D
E
A
B
D
E
C
5 years
4 years
5 years
1 year
3 years
Total p-y = 18
Incidence= (3/18)x1000 = 166.67 per 1000 person-years

15. •INCIDENCE DENSITY
•FORCE OF MORBIDITY

16. The denominator of an incidence - 3
• Number of people free of the disease in the
population at risk at the beginning of the
period.
• INCIDENCE PROPORTION
• CUMULATIVE INCIDENCE

17. Cumulative incidence rate
= (274/118539)x1000
= 2.31 cases per 1000 women

18. When calculating incidence, those having
the disease in the population at the start or
baseline need to be identified and excluded
from the denominator

21. Attack rate
• The term “attack rate” is often used instead of
incidence during a disease outbreak in a
narrowly-defined population over a short
period of time.
• The attack rate can be calculated as the
number of people affected divided by the
number exposed.

22. Secondary Attack Rate
• The number of cases of an infection that occur
among contacts within the incubation period
following exposure to a primary case in relation to
the total number of exposed contacts; the
denominator is restricted to susceptible contacts
when these can be determined.
• The secondary attack rate is a measure of
contagiousness and is useful in evaluating
control measures.

23. Case fatality
• the proportion of cases with a specified
disease or condition who die within a
specified time.
• usually expressed as a percentage.
• Case fatality is a measure of disease severity

25. The relationship between incidence
and prevalence
PREVALENCE = INCIDENCE X Average DURATION of disease
If prevalence is low and does not vary significantly with time.

26. Jan 1,
2013
Dec 31,
2013
Population at risk =200
(on 1st July, 2013)

27. A cohort of 100 people at risk of a disease was followed
from 2012 to 2014. Following information is available.
Case no Disease start date Disease end date
1 Nov 2012 Feb 2013
2 Mar 2013 Dec 2013
3 Oct 2013 Feb 2014
4 Aug 2013 Nov 2013
5 Sep 2013 Dec 2013
6 Jan 2013 June 2013
7 Oct 2013 Mar 2014
8 Oct 2012 Dec 2012
9 Feb 2013 Sep 2013
10 Mar 2013 May 2104
Calculate the incidence rate for the period Jan 2013 to Dec 2013.