There are a few reasons thay Tammy's sample may not be valid. Firstly, she had a relatively small sample size, meaning that she had a potentially large margin of error. Additionally, she was asking students from her math class, which could also have created a bias simply due to the fact that they were currently doing math. In order to survey a more valid sample, she should consider student of other grade levels and from other classes to ensure that she has the most information possible and to create variation in her sample.
