To see if this was the case, we worked through an activity in the CollegeBoard's Curriculum Module on Random Sampling and Random Assignment. There are 14 subjects listed on cards. Each card has the subject's name, gender and IQ score. Students shuffled the cards and split the cards into two piles of 7 cards. Cards in the left pile represented the students that were randomly assigned to watch an episode of Sponge Bob Squarepants. Cards in the right pile represented the students that were randomly assigned to the drawing group.
Next, students calculated the difference in the proportion of females in the two piles (Sponge Bob group - Drawing group) and the difference in the average IQ scores in the two piles (Sponge Bob group - Drawing group). Each pair did this a total of five times and then we recorded our results on a dotplot. Students noted that there was variability from sample to sample, but that the dots tended to center around 0 for both the difference in the proportions of females and the difference in the mean IQ. Here you can see our class dotplot.