Successfully reported this slideshow.
Upcoming SlideShare
×

# Epidemiology lecture3 incidence

2,539 views

Published on

Epidemiology
Measuring Disease Frequency-Incidence

Published in: Health & Medicine
• Full Name
Comment goes here.

Are you sure you want to Yes No
• Be the first to comment

### Epidemiology lecture3 incidence

1. 1. EPIDEMIOLOGY Lecture 3: Incidence
2. 2. What is incidence? • The rate at which events occur in a population. new
3. 3. Numerator Denominator Multiplier Number of NEW events in a specified period X 10 n The numerator strictly refers only to first events of disease.
4. 4. • Frequency Count • Rate • Proportion
5. 5. • 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
6. 6. 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.
7. 7. 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
8. 8. 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
9. 9. 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
10. 10. •INCIDENCE DENSITY •FORCE OF MORBIDITY
11. 11. 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
12. 12. Cumulative incidence rate = (274/118539)x1000 = 2.31 cases per 1000 women
13. 13. When calculating incidence, those having the disease in the population at the start or baseline need to be identified and excluded from the denominator
14. 14. 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.
15. 15. 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.
16. 16. 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
17. 17. The relationship between incidence and prevalence PREVALENCE = INCIDENCE X Average DURATION of disease If prevalence is low and does not vary significantly with time.
18. 18. Jan 1, 2013 Dec 31, 2013 Population at risk =200 (on 1st July, 2013)
19. 19. 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.