We propose a methodology for estimating the competition effects from entry when firms sell differentiated products. We first derive precise conditions under which Bresnahan and Reiss’s entry threshold ratios (ETRs) can be used to measure competition effects. We then augment the traditional entry model with a revenue equation. This serves to adjust the ETRs by the extent of market expansion from entry, giving unbiased estimates of the competition effects. We apply our approach to seven local service sectors. We find that entry typically leads to significant market expansion, implying that traditional ETRs may substantially underestimate the competition effects from entry.
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