Note that Cheryl wants to find the number of hours seventh grade boys and girls spend on homework each week.
There are multiple causes of a biased sample. Two of the most obvious ones I find are:
1. She's surveying girls, but she is trying to get data on the number of hours all seventh graders (boys and girls) spend.
2. She's surveying 50 girls from the entire school, although she is trying to get data on seventh graders only.
