Examine the fictional experimental study described below:
The National Institutes of Health (NIH) set up a long-range study to determine whether exposing infants to indoor pets (dogs or cats) during childhood affected their development of allergies as older children and young adults. The researchers identified 150 volunteer families with infants between 3-6 months old for their study. For the study, 50 families agreed to keep a single indoor cat for a 3-year period, 50 families agreed to keep a single dog (kept mainly indoors) for a 3-year period, and 50 families kept no pets during this period.
Following the 3-year period, the children were given yearly allergy tests until age 20 to test for the development of allergies. After analyzing the data, researchers found a decreased incidence of allergies in the pet-containing group (with slightly fewer, but statistically significant, allergies occurring in the cat group than the dog group).
What is the dependent variable in this experiment?