Don’t forget the basics. The Behavioral Risk Factor Surveillance System

(BRFSS) survey found that 279 individuals in its 2006 random sample of 2166 college graduates in California said they had engaged in binge drinking 474 CHAPTER 21 What Is a Confidence Interval?

in the past year. We used this finding to calculate confidence intervals for the proportion of all college graduates in California who engaged in binge drinking in the past year. This sample survey may have bias that our confidence intervals do not take into account. Why is some bias likely to be present? Does the sample proportion 12.9% probably overestimate or underestimate the true population proportion?