Cata Mount Inc., a North Carolina firm, produces a part used in a pharmaceutical production process.  Mount hires one of two shipping firms (A and B—names are concealed to protect the innocent) to deliver the part.  To better represent delivery time to its pharmaceutical customers, Mount collects delivery times on 50 randomly selected Chicago shipments by each shipper.  These times are recorded in shiptimes.xls.  Use this information to analyze shipping times for Mount.

1. Use z-values to determine if there are any outlier times in the data set  .  List the outliers and their z-values for each shipper.  Explain to Mount management why these values are unusual by interpreting their z-values.  The manager studied statistics a long time ago and recalls the meaning of mean and standard deviation but not z-values.

2. Suppose that you check with each supplier and learn that all of the outlier shipments were subcontracted to Shipper C (C’s erratic behavior probably merits exposure but this is not the Better Business Bureau we’re analyzing).  Neither A nor B uses C any longer, so omit all of the outlier shipments from each sample and continue the analysis.

3. Which supplier has the fastest average delivery time (round three places past decimal)?  Explain.

4. If we ignore the fact that the mean delivery times differ, which supplier is more consistent with the time for its deliveries  (round three places past decimal)?

5. When customers order the part, they often need to arrange for installation personnel to be present, so they ask Mount to predict the delivery date as accurately as possible.  They desire accuracy rather than speed.  Mount determines the shipper, and then predicts the mean time. Mount’s manager acknowledges that delivery times will vary.  But she believes that Chicago customers will discredit the firm if they perceive that Mount misses its predicted delivery time by a relatively large amount.  She wants to only use one shipper in the future.  Use the shippers’ coefficients of variation to choose the shipper who will produce the best psychological perception that Mount can predict delivery times well.  Explain.