You take a survey of students who attend class on a particular campus @ 9:30AM & find that 25% of those students are parents. Would you feel comfortable concluding that the proportion of all BC students who are parents is also 25%? Why or why not?
No, you would not, since this is very likely to be a biased sample. Since you have only one campus, and one time represented in your sample, if these are correlated with the statistic you are interested in (parenthood) your estimate of 25% and the true overall number might be very different. For example, perhaps parents are less likely to sleep in and miss 9:30am classes, if they have to send their children to school, and so 25% may be an overestimate.