[msgilligan]

There are multiple ways of calculating market share (e.g. units vs dollars or for different time periods) but assuming it is measured in dollars for a quarterly time period, how would you calculate the market share based upon my sample data above?

[oncallthrow]

(company sales in period - company returns for sales made in said period) / (industry sales in period)

[matt_kantor]

With this formula market share for past periods can keep changing arbitrarily far into the future (depending on the companies' return policies).