200702016-6 Confidence IntervalA confi dence interval is a range (interval) that includes a statistical value, usually thepopulation mean.A confi dence interval that is too broad makes it diffi cult to get an idea of where the populationvalue (true value) is located. A narrow confi dence interval, on the other hand, limits thepopulation value and makes it difficult to obtain reliable results. The most commonly usedconfi dence levels are 95% and 99%. Raising the confidence level broadens the confidenceinterval, while lowering the confidence level narrows the confi dence level, but it alsoincreases the chance of accidently overlooking the population value. With a 95% confidenceinterval, for example, the population value is not included within the resulting intervals 5% ofthe time.When you plan to conduct a survey and then t test and Z test the data, you must alsoconsider the sample size, confi dence interval width, and confi dence level. The confidencelevel changes in accordance with the application.1-Sample Z Interval calculates the confidence interval for an unknown population meanwhen the population standard deviation is known.2-Sample Z Interval calculates the confidence interval for the difference between twopopulation means when the population standard deviations of two samples are known.1-Prop Z Interval calculates the confi dence interval for an unknown proportion of successes.2-Prop Z Interval calculates the confi dence interval for the difference between the propotionof successes in two populations.1-Sample t Interval calculates the confi dence interval for an unknown population meanwhen the population standard deviation is unknown.2-Sample t Interval calculates the confi dence interval for the difference between twopopulation means when both population standard deviations are unknown.On the initial STAT mode screen, press 4(INTR) to display the confidence interval menu,which contains the following items.• 4(INTR)1(Z) ... Z intervals (page 6-6-3)2(t) ... t intervals (page 6-6-8)# There is no graphing for confi dence intervalfunctions.6-6-1Confi dence Interval
