Using t Procedures Wisely Using One-Sample t Procedures: The Normal Condition Sample size less than 15: Sample size at least 15: Large samples: Use t procedures if the data appear close to Normal (roughly symmetric, single peak, no outliers). If the data are clearly skewed or if outliers are present, do not use t. The t procedures can be used except in the presence of outliers or strong skewness. The t procedures can be used even for clearly skewed distributions when the sample is large, roughly n ≥ 30.

Can we use t? Determine whether we can safely use a one-sample t interval to estimate the population mean in each of the following settings. The dotplot below shows the amount of time it took to order and receive a regular coffee in 5 visits to a local coffee shop. Below is a stemplot of the force required to pull apart 20 pieces of Douglas fir. Below is a stemplot of the lengths of 23 specimens of the red variety of the tropical flower Heliconia

Need Help? Give Us a Call! A large bank decided to study the call response times in its customer service department. The bank’s goal was to have a representative answer an incoming call within 30 seconds. Below is a histogram of the response times in a random sample of 241 calls to the bank’s customer service center in a given month. What does the graph suggest about how well the bank is meeting its goal? An analysis of the data reveals that = seconds, s x = seconds, and that 41 of the 241 call response times were 30 seconds or more

A humane society wanted to estimate with 95 percent confidence the proportion of households in its county that own at least one dog. (a) Interpret the 95 confidence level in this context The humane society selected a random sample of households in its county and used the sample to estimate the proportion of all households that own at least one dog. The conditions for calculating a 95 percent confidence interval for the proportion of households in this county that own at least one dog were checked and verified, and the resulting confidence interval was 0.417± (b) A national pet products association claimed that 39 percent of all American households owned at least one dog. Does the humane society’s interval estimate provide evidence that the proportion of dog owners in its county is different from the claimed national proportion? Explain. (c) How many households were selected in the humane society’s sample? Show how you obtained your answer.