A paired stimulus preference assessment was conducted using the four toy sets (Fisher et al., 1992). First, each participant
was individually allowed 2-min of free access to each set of toys one at a time. This was done to give the participants
experience with each set of toys and allow them the opportunity to form a preference. Then, to assess preference, toys were
presented two at a time and the participant was asked to ‘‘pick a toy’’. Selection was defined as reaching for or pointing
toward one of the two toys offered. After the child selected a toy they were given 20-s of access. The toy was then removed
and another pair of toys was presented. In order to control for the potential of position bias (e.g., always selecting the toy on
the left), the position of any given toy was systematically altered across presentations so that it was presented on both the
left and right side of the pair. Every combination of two toys was presented and selections were recorded. For each
participant, the two sets of toys selected the most frequently were considered to be the most preferred. All three participants
selected the same two sets of toys (see description of toys below) more often than the competing toy sets. Therefore, these
two sets of toys were used with all three participants.