Diarrhea and colds in child day care centers: impact of various numerator and denominator definitions of illness episodes.
BACKGROUND
Numerators and denominators used to estimate infections' incidence rates (IRs), incidence rate ratios (IRRs), and differences (IRDs) vary. Our objective is to quantify the impact of various common definitions for illness episodes (numerators) and person-time at risk (denominators) in estimating these measures.


METHODS
Data were from a cohort study in which daily occurrence of illness and children's attendance in day care centers were recorded. We compared 4 IR estimates using various definitions of episode and at-risk time units.


RESULTS
IRs for diarrhea and colds were highest using child-days, lowest for diarrhea using child-weeks, and lowest for colds using child-months. The 4 methods led to similar IRRs but considerably different IRDs.


CONCLUSIONS
Incidence rate differences and ratios for infectious diseases can vary by the definition of episode and person-time at risk. This limits the value of the evidence base on which public health recommendations are formulated. Development of a more standard approach to measurement and reporting of IRs is recommended.
Department of Biostatistics and Epidemiology, College of Public Health, University of Oklahoma Health Sciences Center, Oklahoma City, OK 73104, USA.
Mushfiqur R
MR
Tarafder
Hélène
H
Carabin
Theresa W
TW
Gyorkos
Lawrence
L
Joseph
D002656:Child Day Care Centers; D002675:Child, Preschool; D015331:Cohort Studies; D003139:Common Cold / Q000453:epidemiology*; D003967:Diarrhea / Q000453:epidemiology*; D006801:Humans; D015994:Incidence; D011159:Population Surveillance / Q000379:methods*; D011792:Quebec / Q000453:epidemiology; D016032:Randomized Controlled Trials as Topic