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Control Charts

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Researchers at Miami University in Oxford, Ohio, investigated the use of p charts to monitor the market share of a product and to document the effectiveness of marketing promotions. Markt share is defined as the company's proportion of the total number of products sold in a category. If a p-chart based on a company's market share indicates an in-control process, then the company's share in the marketplace is deemed to be stable and consistent over time. In the example given in the article, the RudyBird Diskette Company collected daily sales data from a nationwide retail audit service. The first 30 days of data in the accompanying table (file rudybird.xls) indicate the total number of cases of computer disks sold and the number of RudyBird diskettes sold. The final 7 days of data were taken after RudyBird launched a major in-store promotion. A control chart was used to see if the in-store prommotion would result in special cuase variation in the marketplace.

a. Construct a p chart using data from the first 30 days (prior to the promotion) to monitor the market share for RudyBird Diskettes.

b. Is the market share for RudyBird in control before the start of the in-store promotion?

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