The following table provides the costs for gasoline by month in the United States over recent years and the percentage of the cost due to refining, distribution and marketing, taxes, and crude oil. The table is from the U.S. Department of Energy Web site (http://tonto.eia.doe.gov/oog/info/gdu/gaspump.html). There is some concern that the refining or distribution and marketing percentages of the retail price have shown patterns over time.

(a) Construct separate control charts for the refi ning percentage of the retail price and the distribution and marketing percentage of the retail price. Use control charts for individual measurements. Comment on any signs of assignable causes on these charts.

(b) Construct a control chart for the crude oil percentage of the retail price. Use a control chart for individual measurements. Comment on any signs of assignable causes on this chart. 

(c) Another way to study the data is to calculate refining, distribution and marketing, and tax as costs directly. The costs of these categories might not depend strongly on the crude oil cost. Use the percentages provided in the table to calculate the cost associated with refining and distribution and marketing each month. Construct separate control charts for refining and distribution and marketing costs each month. Use control charts for individual measurements. Comment on any signs of assignable causes on these charts and comment on any differences between these charts and the ones constructed in part (a).

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What We Pay for in a Gallon of Regular Gasoline Retail Price Distribution & Marketing (%) (dollars per gallon) Taxes (%) Crude Oil (%) Mo/Year Refining (%) Jan-00 1.289 7.8 13.0 32.1 47.1 7.5 Feb-00 1.377 17.9 30.1 44.6 12.8 27.3 Mar-00 1.517 15.4 44.6 Apr-00 May-00 28.3 1.465 10.1 20.2 41.4 20.2 1.485 9.2 27.9 42.7 22.2 1.633 25.8 Jun-00 8.8 43.1 13.2 Jul-00 1.551 15.8 27.2 43.8 Aug-00 1.465 15.8 7.5 28.8 47.8 27.2 Sep-00 1.550 15.4 9.0 48.3 Oc-00 1.532 13.7 10.1 27.5 48.6 1.517 Nov-00 10.4 11.8 27.8 50.0 Dec-00 1.443 8.0 17.9 29.2 44.8 Jan-01 1.447 17.8 10.4 29.2 42.7 Feb-01 1.450 17.3 11.0 29.1 42.6 Mar-01 1.409 18.8 9.7 30.0 41.5 36.7 Apr-01 1.552 31.6 4.6 27.1 May-01 1.702 26.4 14.0 24.7 35.0 36.7 Jun-01 1.616 13.2 24.1 26.0 20.0 Jul-01 1.421 10.0 30.0 40.0 41.0 Aug-01 1.421 20.0 9.0 30.0 Sep-01 1.522 18.0 17.0 28.0 37.0 Oct-01 1.315 10.0 20.8 31.9 37.2 Nov-01 1.171 10.0 18.0 36.0 36.0 36.9 Dec-01 1.086 11.7 12.7 38.7 Jan-02 1.107 13.0 11.8 37.9 37.2 11.2 37.7 Feb-02 1.114 12.1 39.1 Mar-02 1.249 19.4 6.1 33.6 40.9 Apr-02 1.397 15.5 13.0 30.1 41.4 1.392 30.2 May-02 11.9 14.2 43.7 13.0 Jun-02 1.382 15.0 30.4 41.6 Jul-02 1.397 15.0 12.6 30.1 42.3 Aug-02 1.396 11.4 13.4 30.0 45.0 Sep-02 1.400 10.8 12.6 30.0 46.7