RELATION OF PROFIT RATE TO INDUSTI 295 with(1) identical entry conditions and(2)an identical industry demand and cost situation, will tend to sell at a lower price and receive a smaller profit. 8 In short, if we hold demand and cos conditions and entry conditions constant, monopoly or effectively allusive oligopoly tends to yield higher profit aggregates and prices in long-run equilibrium than competition or imperfectly-or non- collusive oligopoly Retaining suppositions(1)and(2)above, monopoly or effectively collusive oligopoly will also bring forth a higher excess profit rate on sales. This will be true even where cost and demand conditions are not identical, so long as the opportunity for aggregate profit relative to aggregate sales is the same. The excess profit rate on sales(after deducting all costs, including all paid and imputed interest)should average higher in long-run equilibrium among industries with monop- oly or effectively collusive oligopoly than among others, so far as on the average the relation of industry demand to cost and the conditions of entry are about the same This association should actually hold even in long-run equilibrium only on the average and not in each case. Similarity of demand-cost relations should be found only on the average, as should any similarity of entry conditions. When we leave the static model, the same association should tend to hold on the average through time, but a considerable dispersion of individual profit rates for particular inter- vals could be caused by difference in trends and fluctuations of demand, in the rate of innovation, and so forth. Thus individual industries in the more"competitive' category may have as high as or higher profit rates than individuals in the"monopoly 'category but on the assumption that all other influences on the profit rate f'average-out'' within groups, the group averages over time of excess profit rates on sales should differ in the manner indicated. 9 Average excess profit rates on sales should thus be higher with han without monopoly or effective oligopolistic collusion. This prediction evolves into one that there will be larger profit rates with high seller concentration than with moderate or low seller concentra- tion if we posit a systemati ation between the probability of ORO G s enough lower to forestall entry and thus to maximize the long-run pre American Economic Review, March 1949, pp. 448-64 to or below the competitive level 9. This averaging-out'' should also presumably apply to any effects of any other differences in market structure which may have an independent influence on profit rates