One-Sided Confidence Interval A one-sided claim about a population proportion is a claim that the proportion is less than (or greater than) some specific value. Such a claim can be formally addressed using a one-sided confidence interval for p, which can be expressed as p 6 pn + E or p 7 pn - E, where the margin of error E is modified by replacing za>2 with za. (Instead of dividing a between two tails of the standard normal distribution, put all of it in one tail.) Use the data given in Exercise 13 “OxyContin” to construct a one-sided 95% confidence interval that would be suitable for addressing the claim that the rate of headaches among OxyContin users is less than 10%.