Costs are rising for all kinds of medical care. the mean monthly rent at assisted-living facilities was reported to have increased 17% over the last five years to $3486 (the wall street journal, october 27, 2012). assume this cost estimate is based on a sample of 120 facilities and, from past studies, it can be assumed that the population standard deviation is $650. to construct a confidence interval for the population mean monthly rent, do we use the z-value or the t-value?