Example: Person-Time Incidence - Pediatric Cardiomyopathy
Lead Author(s): Jeff Martin, MD
Study of Pediatric Cardiomyopathy
Lipshultz 2003 Study presents:
- 467 cases of cardiomyopathy in registry of 38 centers (New England, Southwest) 1996 - 1999
Numerator
Numerators: Based on patients who received a diagnosis of cardiopathy between 1996 - 1999
Denominator
Denominator: Population estimates (incidence-rate denominators) are taken from the 1990 U.S. Census,
- with an in- and out-migration algorithm applied to obtain year-specific population estimates.
Annual Incidence
Overall annual incidence: 1.13 per 100,000 children
- Incidence in New England: 1.44 per 100,000 children
- Incidence in Southwest: 0.98 per 100,000 children
Calculating annual rates is a common practice as many statistics are gathered annually. The way these rates are often reported might lead you to think they are proportions as they are frequently described as per some number of persons without making explicit that it is really per some number of person-years.
- The years are buried in the word annual.
- Some investigators are more precise and will specify rates as per person-years.
Reporting Cancer Rates
The choice of the person-time denominator is usually determined by a desire to have at least one integer to the left of the decimal point. So events that are relatively rare, such as the example of pediatric cardiomyopathy above, are often reported per 100,000 person-years. For the same reason cancer rates are usually reported per 100,000 person-years.
References
Lipshultz, S. E., Sleeper, L. A., Towbin, J. A., Lowe, A. M., Orav, E. J., Cox, G. F., et al. (2003). The incidence of pediatric cardiomyopathy in two regions of the United States. N Engl J Med, 348(17), 1647-1655.