The regulatory debate concerning high-frequency trading (HFT) emphasizes the importance of distinguishing different HFT strategies and their influence on market quality. Using data from NASDAQ-OMX Stockholm, we compare market-making HFTs to opportunistic HFTs. We find that market makers...

Volatility extensions in closing auctions are designed to improve the efficiency of the closing price. We hypothesize that the channel for the efficiency increase is that extensions improve market integrity and investor trust in the auction mechanism. We confirm that the introduction of a...