[scottcodie]

The more people you survey, the higher the confidence that the questionnaire is representative of the entire population, or the 'confidence interval'. Since the population was randomly sampled & non-responses don't correlate to the topic questions, you can assume that the 90 students are representative of the entire population with +- some confidence interval.

[Kon-Peki]

Yes, but we know neither the size of the population nor the confidence interval had the full 120 students responded.

Secondly, non-responses may actually correlate to the topic questions! For all anyone knows, students may not respond because

- too busy studying

- too busy partying

- too busy with intramural sports

- too disillusioned, feeling that it will not make a difference.

Etc