Every fifth student who walked into the middle school was surveyed to determine his or her favorite subject. Out of 150 students, 90 students stated that they like math class best. The staff concluded that about 60% of students like math class best. Is the sample method valid? Justify your answer.
a)

The conclusion is invalid. This is a biased voluntary response sample.


b)

The conclusion is invalid. This is a biased convenience sample.


c)

The conclusion is valid. This is an unbiased systematic random sample.


d)

The conclusion is valid. This is an unbiased simple random sample.