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Epidemiology

Measuring Disease Frequency-Incidence

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- 1. EPIDEMIOLOGY Lecture 3: Incidence
- 2. What is incidence? • The rate at which events occur in a population. new
- 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. • Frequency Count • Rate • Proportion
- 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. 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. 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. 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. 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. •INCIDENCE DENSITY •FORCE OF MORBIDITY
- 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. Cumulative incidence rate = (274/118539)x1000 = 2.31 cases per 1000 women
- 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. 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. 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. 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. 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. Jan 1, 2013 Dec 31, 2013 Population at risk =200 (on 1st July, 2013)
- 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.

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