Data analysis. An iterative process was used to develop the prediction rule for accurately classifying whether a study child had a viral URI (19, 20) . Ratings for each measured symptom for the first 4 d of the illness were coded into two variables. The first measure of each symptom was whether it was present, corresponding to scores of 0 versus scores of 1-3. The second measure was whether the symptom was present and of "significant severity," corresponding to scores of 0 -1 versus 2-3. With this procedure, 96 different variables defining the presence or absence of symptoms were created (12 symptoms ϫ 2 measures/symptom ϫ 4 d of assessment). The rate of presence of each symptom measure in children with a PCR confirmed viral URI and those with a negative PCR ("no URI") was then compared using 2 tests. Those symptom measures for which rates in the two groups (viral URI or no URI) were different (p Ͻ 0.20) were classified as candidate symptom measures and included in the next step. For symptoms in which both severity measures (e.g., any runny nose and significant runny nose) were different in patients with and without viral URI, only the measure with the more significant difference was included. In addition, candidate symptom measures in which greater than 10% of parents indicated "can't tell" regarding the presence of the symptom were excluded.