a telephone service representative believes that the proportion of customers completely satisfied with their local telephone service is different between the midwest and the west. the representative's belief is based on the results of a survey. the survey included a random sample of 1280 midwestern residents and 1380 western residents. 50% of the midwestern residents and 54% of the western residents reported that they were completely satisfied with their local telephone service. find the 90% confidence interval for the difference in two proportions. step 1 of 3: find the critical value that should be used in constructing the confidence interval.