Tammy's sample may not be valid because there could be a biased selection of students who participated in her survey. For example, Tammy may have only surveyed students in her math class, which could have skewed the results towards math being the favorite subject. Additionally, the sample size may not be large enough to represent the entire student population at her school accurately.
Furthermore, Tammy's survey questions may have been leading towards math being the favorite subject. For example, if Tammy asked students to rate their favorite subject from 1 to 5, and only listed math, English, and history as options, students who may have had a different favorite subject such as science or art would not have been able to select it.
Therefore, while Tammy's conclusion that math is the favorite subject may be true for the students she surveyed, it may not be valid for the entire student population at her school. A larger and more diverse sample size would be needed to make a more accurate conclusion about the favorite subject of the students at her school.