One-Sided Confidence Interval A one-sided confidence interval for p can be expressed as p , 1 E or p . 2 E, where the margin of error E is modified by replacing with za. If Air America wants to report an on-time performance of at least x percent with 95% confidence, construct the appropriate one-sided confidence interval and then find the percent in question. Assume that a simple random sample of 750 flights results in 630 that are on time.