Tom and Juanita conducted an experiment to test the effect of age on difference threshold in children 5 years of age and younger. They created an experimental setup that included cards mounted with 12 pairs of paint chips of the same hue but of various shades. The difference between the first pair was fairly obvious, but the difference between each subsequent pair decreased. The last few pairs of chips seemed almost identical. The process that Tom and Juanita used was to show the pairs to a child and to note the smallest amount of difference the child could detect when asked to identify the darker chip on each card. They tested 3 children from each of these age groups: 2, 2.5, 3, 3.5, 4, 4.5, 5. 1. What is the dependent variable?