Disease Incidence Rate
A Disease Incidence Rate is an rate for an disease incidence measure (number of new cases of a disease occurring during a given period as a proportion of the number of people in the population).
- Example(s):
- CONV-19 Incidence Rate (on 2020-02-26).
- See: Cumulative Incidence, Epidemiology, Medical Condition, Denominator Data.
References
2020
- (Wikipedia, 2020) ⇒ https://en.wikipedia.org/wiki/Incidence_(epidemiology)#Rate Retrieved:2020-2-27.
The incidence rate is a measure of the frequency with which a disease or other incident occurs over a specified time period. [1][2] When the denominator is the product of the person-time of the at risk population, it is also known as the incidence density rate or person-time incidence rate.[3] In the same example as above, the incidence rate is 14 cases per 1000 person-years, because the incidence proportion (28 per 1,000) is divided by the number of years (two). Using person-time rather than just time handles situations where the amount of observation time differs between people, or when the population at risk varies with time.[4] Use of this measure implies the assumption that the incidence rate is constant over different periods of time, such that for an incidence rate of 14 per 1000 persons-years, 14 cases would be expected for 1000 persons observed for 1 year or 50 persons observed for 20 years.[5]
When this assumption is substantially violated, such as in describing survival after diagnosis of metastatic cancer, it may be more useful to present incidence data in a plot of cumulative incidence, over time, taking into account loss to follow-up, using a Kaplan-Meier Plot.
Consider the following example. Say you are looking at a sample population of 225 people, and want to determine the incidence rate of developing HIV over a 10-year period:
- At the beginning of the study (t=0) you find 25 cases of existing HIV. These people are not counted as they cannot develop HIV a second time.
- A follow-up at 5 years (t=5 years) finds 20 new cases of HIV.
- A second follow-up at the end of the study (t=10 years) finds 30 new cases.
If you were to measure prevalence you would simply take the total number of cases (25 + 20 + 30 = 75) and divide by your sample population (225). So prevalence would be 75/225 = 0.33 or 33% (by the end of the study). This tells you how widespread HIV is in your sample population, but little about the actual risk of developing HIV for any person over a coming year.
To measure incidence you must take into account how many years each person contributed to the study, and when they developed HIV. When it is not known exactly when a person develops the disease in question, epidemiologists frequently use the actuarial method, and assume it was developed at a half-way point between follow-ups. In this calculation:
- At 5 yrs you found 20 new cases, so you assume they developed HIV at 2.5 years, thus contributing (20 * 2.5) =50 person-years of disease-free life.
- At 10 years you found 30 new cases. These people did not have HIV at 5 years, but did at 10, so you assume they were infected at 7.5 years, thus contributing (30 * 7.5)= 225 person-years of disease-free life. That is a total of (225 + 50)= 275 person years so far.
- You also want to account for the 150 people who never had or developed HIV over the 10-year period, (150 * 10) contributing 1500 person-years of disease-free life.
That is a total of (1500 + 275) = 1775 person-years of life. Now take the 50 new cases of HIV, and divide by 1775 to get 0.028, or 28 cases of HIV per 1000 population, per year. In other words, if you were to follow 1000 people for one year, you would see 28 new cases of HIV.
This is a much more accurate measure of risk than prevalence.
2008
- (Upton & Cook, 2008) ⇒ Graham Upton, and Ian Cook. (2008). “A Dictionary of Statistics, 2nd edition revised." Oxford University Press. ISBN:0199541450
- Incidence Rate: The number of new cases of a disease occurring during a given period as a proportion of the number of people in the population. It is usually expressed as cases per 1,000 or 100,000 per annum. See AGE-SPECIFIC RATE; MORBIDITY RATE; MORTALITY RATE; SEX-SPECIFIC RATE.
- ↑ Hargrave, Marshall. "What Does the Incidence Rate Measure?" (in en). https://www.investopedia.com/terms/i/incidence-rate.asp. Retrieved 2019-09-30.
- ↑ Monson, Richard R. (1990-04-25) (in en). Occupational Epidemiology, Second Edition. CRC Press. pp. 27. ISBN 978-0-8493-4927-0. https://books.google.be/books?id=Cx9dcDdO1hwC&pg=RA1-PA27#v=onepage&q&f=false.
- ↑ Last, John M., ed. (2001). A Dictionary of Epidemiology (4 ed.). New York, NY: Oxford University Press. ISBN 978-0-19-514169-6.
- ↑ "Quantifying diseases in populations". Epidemiology for the Uninitiated (4th ed.). BMJ. 1997. ISBN 978-0-7279-1102-5. http://www.bmj.com/about-bmj/resources-readers/publications/epidemiology-uninitiated/2-quantifying-disease-populations.
- ↑ Dunn, Olive Jean; Clark, Virginia A. (2009). Basic statistics: a primer for the biomedical sciences (4th ed.). Hoboken, N.J.: John Wiley & Sons. pp. 3–5. ISBN 9780470496855. https://books.google.co.uk/books?id=aMNO88xHGqsC&pg=PA3. Retrieved 9 May 2016.