Question: 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. Market share in 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 (see the 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 would result in special cause variation in the marketplace.

Cases Sold Before the Promotion

Day Total RubyBird

1 154 35

2 153 43

3 200 44

4 197 56

5 194 54

6 172 38

7 190 43

8 209 62

9 173 53

10 171 39

11 173 44

12 168 37

13 184 45

14 211 58

15 179 35

16 177 56

17 143 43

18 200 69

19 134 38

20 192 47

21 155 45

22 135 36

23 189 55

24 184 44

25 170 47

26 178 48

27 167 42

28 204 71

29 183 64

30 169 43

31 201 92

32 177 76

33 205 85

34 199 90

35 187 77

36 168 79

37 198 97

Source: Extracted from C.T. Crespy, T.C. Krehbiel, and J.M. Stearns, “Integrating Analytic Methods into Marketing Research Education: Statistical Control Charts as an Example,” Marketing Education Review, 5, Spring 1995, pp. 11-23.

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

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

c. On your control chart, extend the control limits generated in ( b ) and plot the proportions for days 31 through 37. What effect, if any, did he in-store promotion have on RubyBird’s market share?