The Pet Rescue Club at a local high school is considering selling candy bars as a way of raising funds to donate to the local animal shelter. Club members wondered how many candy bars each person should be able to sell on average. They knew that members of the school band had sold the same candy bars for fundraising in the past, so they asked for information from the band director.
The band director, who fancies himself a bit of a statistician on the side, provided the stemplot below which shows the sales results of a random sample of $18$ band members who had sold candy bars.
The President of the Pet Rescue Club had planned to use the information provided by the band director to construct a one-sample $t$-interval to estimate the mean number of candy bars each person could be expected to sell. Then she realized that one of the necessary conditions for inference had not been satisfied.
Which condition was not satisfied?