Unformatted text preview: rgin of error: 1.) Increase sample size 2.) Use a lower level of confidence (smaller C) 3.) Reduce x x z* x n under certain circumstances: Be careful!!!! You can only use the formula Data must be an SRS from the population. Do not use if the sampling is anything more complicated than an SRS. Data must be collected correctly (no bias). The margin of error covers only random sampling errors. Undercoverage and nonresponse are not covered. Outliers can have a big effect on the confidence interval. (This makes sense because we use the mean and standard deviation to get a CI.) You must know the standard deviation of the population, x . 3 EXAMPLE 1: A questionnaire of spending habits was given to a random sample of college students. Each student was asked to record and report the amount of money they spent on textbooks in a semester. The sample of 130 students resulted in an average of $422 with standard deviation of $57. a)
Give a 90% confidence interval for the mean amount of money spent by college students on textbooks. b)
Is it true that 90% of the students...
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