The four-firm concentration ratio is calculated by adding the market shares of the four largest firms: in this case, 30 + 20 + 20 + 20 = 90. This concentration ratio would can be considered especially high, because the largest four firms have 90% of the market. Herfindahl index is calculated by squaring the market share of each firm competing in a market and then summing the resulting numbers. In this case market share of the 1st firm is 30% (30/(30+20+20+20+10)*100%=30%), 2nd, 3rd, 4th - 20%, 5th - 10%. Herfindahl index is"30*30+(20*20)*3+10*10=2,200"
Based on this calculations, the watch industry in Geneva is a moderately concentrated marketplace, (less than 2,500), but taking into account the four-firm concentration ratio the market is oligopolistic.