st: Index

["Pavlos C. Symeou" <p.symeou@gmail.com>]

Dear Statalisters,

Happy new year to all. I'd like to ask for your assistance 
regarding the following:

I have a number of companies for a number of years and each company 
sells its products in a number of markets in a year. For each 
market, I have the value of its sales (a proportion relative to the 
overall sales for the company in a  year). I am interested in 
calculating an index for each company-year as following:

1) for each company-year I must compare each market with each 
other. If the two markets are the same I take their product and 
multiply it by 1. If the two markets are not the same, I take their 
product and multiply them by 2. The index should be the sum of all 
products.

From the example data below, the Index for Firm 1 in Year 1993 
should be:
(0.4 * 0.2 * 1) + (0.4 * 0.4 * 2) + (0.2 * 0.4 * 2) = 0.56

DATA

Firm   Year      Market    Market_sales    Index
1         1990    1              1
1         1991    1              1
1         1992    2              .3
1         1992    2             .1
1         1992    3              .15
1         1992    2             .15
1         1992    3             .15
1         1992    4             .15
1         1993    4              .40
1         1993    4             .20
1         1993    5             .40
2         1990    1              1
2         1991    1              1
2         1992    2              .3
2         1992    2             .1
2         1992    3              .15
2         1992    2             .15
2         1992    3             .15
2         1992    4             .15
2         1993    4              .40
2         1993    4             .20
2         1993    5             .40

Thanks,

Pavlos

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