Example: Incidence Reporting - What Is Missing?

Lead Author(s): Jeff Martin, MD

Measures Loosely Called Incidence

Number of Events (E)

Count of the number of events (E) Incidence requires that we know how many events occurred (E) during what time period (T) among how many persons (N).

Number of Events During Time Period (E/T)

Count of the number of events during some time period (E/T) Traffic accidents in San Francisco County for the past year has an implied number of persons, which is the average county population during the year, so by naming a geographic location a number of persons at risk is implied.

Number (N) of Persons

In the above examples , neither explicitly includes the number of persons (N) giving rise to the events

These inferences about N and T can be reasonable depending upon the context, but the point we are making is that all three E, N, and T need to be accounted for, preferably explicitly, in order to have a measure of incidence.

Example of Chickenpox Rates

Report in the SF Chronicle, Thursday, September 18, 2003 (09-18) 13:51 PDT ATLANTA (AP) --

CDC: Chickenpox rates drop in four states as inoculations become common

The number of chickenpox cases in four states dropped more than 75 percent as inoculations became more common in the last decade, according to a federal study released Thursday.

The total number of cases in Illinois, Michigan, Texas and West Virginia dropped from about 102,200 in 1990 to about 24,500 in 2001, the Centers for Disease Control and Prevention said.

At the same time, the percentage of infants receiving chickenpox shots rose from less than 9 percent in 1996 to as much as 83 percent in 2001, the CDC said.

Analysis of Chickenpox Report

In this example,
  1. The number of events (E) is given.
  2. The time period (T) is described (one year at two points in time)
  3. A population of persons is specified (four states).
The story says that the number of cases dropped more than 75% and that is perfectly accurate, but the headline says that rates dropped. Since the two one-year incidence periods are 11 years apart, it is a reasonable bet that the population of the four states changed during 11 years. So the press release is probably not qualitatively incorrect (unless those 4 states lost a lot of population), but it would have been even more informative if the incidence rates rather than the counts had been reported. This is information knowable from census data—a good use of census data.

To take it a step further, since chicken pox is largely a disease of infants, it would be even more informative to know what had happened to the size of the population of infants during that period and what the rates were among infants. It is perhaps not so clear that the infant population size increased. If not, then the report may be overstating the change in chickenpox rates among infants.