Electronic Limit Order Books, Dealer/Specialists, And .

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Electronic Limit Order Books, Dealer/Specialists,and Inter-Market Competition on NASDAQMichael A. GoldsteinBabson CollegeEmail: [email protected] V. ShkilkoUniversity of MississippiEmail: [email protected] F. Van NessUniversity of [email protected] A. Van NessUniversity of [email protected] version: February 20041

Electronic Limit Order Books, Dealer/Specialists,and Inter-Market Competition on NASDAQAbstractThis study analyzes execution costs and competition for order flow between theNASDAQ Stock Market and five other trading venues that trade NASDAQ-100securities (QQQ). We find that NASDAQ controls the most market share, although it hasboth the inside bid and ask only 50.24% of the time. These shares are much smaller thansimilar proportions reported for the NYSE, indicating a greater level of quote and orderflow competition on NASDAQ. In addition, the dealer/specialist trading venues(NASDAQ, AMEX, and the Chicago Stock Exchange) show a decreasing percentage oftrades from large to small trades, while trades through electronic limit order book systems(Archipelago and Island) exhibit an increasing percentage of trading from large to smalltrades. We find that both effective and realized spreads are smallest for the electroniclimit order book systems and larger for the dealer/specialist systems. We also findevidence that order flow fragmentation and competition has hurt the markets as theNBBO spreads appear crossed 2.24% of the time and locked 12.43% of the time. Using amultinomial logistic regression, we find that all non-NASDAQ market centers have alower likelihood of executing a trade as compared to NASDAQ itself, although thelikelihood increases if the quotes are more competitive. Our findings suggest that extantfindings on inter-market competition for NYSE-listed securities are not a result of theNYSE being the primary market, but instead due to differences in market structure.2

Traders seeking to trade NASDAQ-listed stocks have a variety of choices inlocations on which they can trade. Currently, NASDAQ stocks quote and trade on sixdifferent venues: NASDAQ, the American Stock Exchange (AMEX), the CincinnatiStock Exchange (CSE), the NASD Alternative Display Facility (ADF), the ChicagoStock Exchange (CHX), and the Pacific Stock Exchange (PSE). These markets allcompete for order flow, not only on the basis of price and quotes, but also cost tobrokers.1 While competition and diversity of market types provides greater choice fortraders and may lead to better prices, it may also lead to fragmentation and the worseningof markets. However, increased competition and fragmentation may lead to frequentlocked or crossed markets.2A recent study by Barclay, Hendershott, and McCormick (2003) examinescompetition between ECNs and market makers (they do not examine competition acrossdifferent market centers), the effects of competition across markets centers have beenmostly studied for NYSE-listed stocks. Studies of competition in NYSE-listed securitiesinclude Blume and Goldstein (1992), Lee (1993), Battalio, Greene, and Jennings (1997),Bessembinder and Kaufman (1997), Bessembinder (2003), and Lipson (2004). Thesestudies examine regional and third market execution for NYSE listed securities. Untilrecently, it has been difficult to examine the different markets for competing prices forNASDAQ stocks. Increased fragmentation, acquisitions and reporting arrangements now1In addition, there is competition within the NASDAQ stock market itself, with multiple dealers, ECNs andlimit order traders competing for order flow.2According to Dean Furbish, Executive Vice President, NASDAQ Transaction Services, the issuesassociated with locked and crossed markets “refers to the relationships between SuperMontage and entitiesthat are outside of it” (Schmerken, 2003).1

allow an examination of the effects of competition and fragmentation within theNASDAQ market system on prices, quotes, and order flow.The purpose of this study, therefore, is to examine the competitive nature of theNASDAQ market. Although most market makers and electronic crossing networks(ECNs) post quotes and report trades through NASDAQ, some, such as Instinet, use theADF. On the other hand, the largest ECN, Island, posts trades and quotes through theCincinnati Stock Exchange. Archipelago, another electronic limit order book system,posts trades and quotes through the Pacific Stock Exchange. While AMEX is a specialistsystem, Chicago is a competing specialist system. These different reporting venues, i.e.,ADF, AMEX, Island (Cincinnati), Archipelago (Pacific) and Chicago, allow us a glimpseinto the inter-competitive nature of NASDAQ. Additionally, we examine issues oflocked and crossed markets, and determinates of order routing.Overall, we find that there is substantial fragmentation of order flow forNASDAQ-listed stocks. Examining the NASDAQ-listed common stocks that comprisethe NASDAQ-100 (QQQ) index from April to June 2003, we find that a little less thanhalf (48%) of all trades occur away from the NASDAQ. Almost 27% of all trades occuron one of the two electronic limit order book systems (Island and Archipelago).However, these results are most prominent for smaller size trades. Over 87% of all blocktrades are still done on NASDAQ itself. While these results are consistent with those inBessembinder (2003) for NYSE-listed securities, we find that NASDAQ’s market sharein terms of volume (62%) is much smaller than that the 85% reported in Bessembinder(2003) for the NYSE, indicating that the market for NASDAQ-listed securities is lesscentered on the primary market. This may be because costs on NASDAQ are higher:2

with the exception of AMEX, we find that NASDAQ has higher effective and realizedspreads than its competitors. NASDAQ is also not particularly competitive in terms ofquote behavior. While it matches the best quote on at least one side of the market about90% of the time, it only matches both sides about half the time. Notably, NASDAQ isalone at quoting the best bid or the best ask only 11% of the time, and is alone at quotingboth sides of the market less than 1% of the time. About 10% of the time, NASDAQquotes neither the best bid or the best ask. Only 2.9% of all of NASDAQ’s volumecomes during this time. There is significant evidence of preferencing, however. Chicagoquotes neither the best bid or the best ask almost all the time (96%); even so, it receives61% of its volume during these periods. Strikingly, we find significant instances oflocked or crossed markets, indicating that the competition across these markets createssignificant instances of market failure.These results contrast with those found in Bessembinder (2003) for the NYSE.While the NYSE matches at least one side of the quote over 99% of the time, it matchesboth sides of the quote almost 90% of the time, much more than NASDAQ does forNASDAQ-listed securities. Even more important, the NYSE is three to four times aslikely to be alone at the best quote than NASDAQ, and is almost never at neither the bestbid or the best ask. In addition, the NYSE has lower effective and realized spreads thanits competitors.Collectively, these results indicate that the primary market is a much less effectivecompetitor for NASDAQ-listed stocks than for NYSE-listed stocks. Overall, there issubstantial competition for order flow from one or more of the other trading venues onNASDAQ.3

Section I of the paper gives a brief history of the trading venues, while section IIdescribes our sample and provides market share information. Section III describes ourtrading cost results, while section IV examines quote competition for NASDAQ-listedstocks. Section V examines trading activity. Section VI examines the results offragmentation by examining locked and crossed markets. Section VII examines thedeterminates of the order routing mechanism. Section VIII concludes.I.History of Trading VenuesEach of the six NASDAQ trading venues have recently had significant changesthat affect their order flow and quoting characteristics. Most of these changes haveoccurred as a result of competitive pressures in the market. As a result of these changes,the disseminated data in TAQ is now more informative about the competitive landscapefor trading NASDAQ-listed securities. In particular, these changes now allow for theidentification of trading on certain electronic limit order books.For example, beginning in March of 2002, Island3, the largest ECN on NASDAQ,began reporting trades to the Cincinnati Stock Exchange. In October of 2002, Island alsobegan routing quotes through the Cincinnati Stock Exchange. Island states that thesemoves were for cost savings, as the Cincinnati Stock Exchange charges less thanNASDAQ to report trades and quotes.Also in October 2002, NASDAQ introduced the Alternative Display Facility(ADF). The ADF is a quotation collection, trade comparison, and trade reporting facilityoperated by the National Association of Securities Dealers Inc. (NASD) for its members.3Island and Instinet merged in 2002. However, Instinet trades and quotes are reported on the ADF.4

The ADF was approved in July 2002 by the SEC as a pilot project (as outlined in ruleSR-NASD-2001-90) proposed by NASD for a period of nine months (July 24, 2002through April 24, 2003). Currently, Instinet trades and quotes are reported through theADF.In early 2003 the Pacific Stock Exchange began disseminating trades and quotesin NASDAQ stocks.4 The Archipelago Stock Exchange uses the Pacific Exchange as themarket center in which it reports the trades and quotes for the NASDAQ stocks in whichthey are making a market.Additionally, two other venues (markets) trade NASDAQ-listed securities5, theChicago Stock Exchange (CHX) and the American Stock Exchange (AMEX). TheChicago Stock Exchange employs a competing specialist system, in some ways similar tothe competing dealer system on NASDAQ. Chicago specialists handle a number ofNASDAQ stocks and have traded some NASDAQ stocks since 1987. Also, AMEXspecialists post quotes and execute trades for Nasdaq stocks.II.SampleThe study considers a sample of 100 NASDAQ-listed common stocks comprisingNASDAQ-100 market index (QQQ).6 The time period for the study is the second quarter4The phase-in of NASDAQ stocks on the Pacific Exchange, by Archipelago, began in February 2003 andwas completed in early April 2003.5SuperMontage was launched by NASDAQ in October 2002. Currently SuperMontage disseminatestrades and quotes through NASDAQ (shown as a “T” in the TAQ database).6We begin with 100 stocks, but 99 have trades and quotes in all three months of our sample. An analysisof the results does not change if this stock is removed from the sample. The stock of issue is USAI (USANetworks) which joined with Vivendi Universal (May 7, 2003), a joint venture was spun off and USANetworks was renamed USA Interactive (and the ticker of that company become IACI).5

of 2003 (April, May and June 2003).7 The data for the study was extracted from Tradeand Quote (TAQ) database provided by the NYSE. The sample is restricted to includeonly quotes and trades that occurred during regular trading hours (9:30 a.m. to 4:00 p.m.EST). Additionally, certain filters are applied to the data to remove observations thatcould be subject to errors. In particular, trades and quotes were omitted if the TAQdatabase indicates that they were out of time sequence or involved either an error or acorrection. Quotes were also omitted if either ask or bid price was equal to or less thanzero. Finally, certain trades were omitted, in case the price or volume was equal to orless than zero.Descriptive statistics for the sample are provided in Table I. Panel A of Table Ishows the market share as a percentage of trades, panel B shows the market share as apercentage of volume, while panel C shows the percentage of dollar volume. NASDAQexecutes a majority (51.59%) of trades in our sample. The two electronic limit orderbook systems, Island, which reports through the Cincinnati Stock Exchange, andArchipelago, which reports through the Pacific Stock Exchange, completed 17.12% and19.31% of the trades, respectively, while the two specialist systems (AMEX and ChicagoStock Exchange) executes only 0.68% together. The ADF executes the remaining11.30% of the trades. Since the AMEX and CHX execute an infinitesimally smallnumber of trades in NASDAQ-listed securities one would expect that, they do not havesubstantial influence on the markets in those securities.7The move by Archipelago to disseminate trades and quotes through the Pacific Stock ended at thebeginning of our sample (they finished the switch on April 11th, 2003). An analysis of only May and Juneyields quantitative similar results as those reported here with all three months.6

However, these results vary across the size of the trade. We divide our sampleinto four categories: large trades (those exceeding 10,000 shares), trades from 5,001 to9,999 shares, trades from 501 to 5000 shares, and small trades (less than 500 shares).NASDAQ market share of large trades is 91.47%, while its market share for small tradesis only 48.34% (Table 1, Panel B). NASDAQ shows a decreasing amount of thepercentage of trades from large to small – and this is also seen for AMEX and Chicago,the two specialist systems. Conversely, the two electronic limit order book systems –Island (CSE) and the Archipelago (PSE) – show an increasing percentage of trades fromlarge to small. Somewhat surprisingly, the ADF also does not repeat the NASDAQ’spattern and executes more small trades (12.09%) than large ones (1.73). Panel C reportsthe market shares as a percentage of dollar volume, and the results are similar to thosepresented in panels A and B. The differences in the trade size categories is statisticallydifferent for all markets for each panel.One very clear pattern emerges from Table I, which is consistent in panel Athrough panel C. The dealer/specialist markets (NASDAQ, AMEX, and the ChicagoStock Exchange) all execute a larger percentage of large trades than small trades, incontrast to the electronic limit order book markets (CSE and PSE), which do the reverse.These differences support the arguments of Blume (2001) and Harris (1993), differentmarkets develop to serve different types of traders needs. In particular, it appears thatsmall traders prefer the advantages of an electronic limit order book system, while largerorders still require the human intervention that comes with dealer/specialist systems.To verify that some markets do not specialize in trading only some of the stocksof our sample, we examine the location of trades and quotes for each of the stocks in our7

sample. An analysis of the data indicates that the Cincinnati Stock Exchange, ADF,Pacific, and NASDAQ all have trades and quotes for the 100 stocks, while AMEX onlyshows trades for 98 of the stocks, and quotes for all 100. The Chicago Stock Exchange ismaking a market in only 80 of the 100 stocks in the sample.III.Trading CostsWith NASDAQ garnering around 50% of the total order flow in NASDAQstocks, and the remaining 50% of trading occurring on other venues, the question remainsat to how much these other venues compete on price or execution quality. We use threemeasures of trade based execution costs: the effective half-spread, price impact, andrealized half-spread.8 To compute these trade based measures, we use the trade from theparticular trading venue, and calculate the National Best Bid and Offer (NBBO) at thetime the trade occurred. The NBBO is not given the TAQ database and must bereproduced from the data. There are instances in which the NBBO indicates that themarket is crossed (bid price is greater than the ask price) or locked (bid and ask are thesame price).9 In the analysis of trade based execution cost measures, we only use NBBOquotes in which the NBBO is greater than zero.10 The effective half-spread, that can bedefined for security i at time t as:8We use trade base execution costs so that we can compute the current NBBO for each stock, and see theexecution cost for each market is at the time of a trade. Computing quoted spread might be misleading, assome of the trading venues might only be competitive for prices when they trades to make, and hence arenot competitive all the time.9An analysis of locked and crossed markets is undertaken in section VI.10An analysis of BBO quotes in which the market is crossed or locked does not indicate evidence of stalequotes. The quotes that lock or cross the market are not outstanding for more than 5 minutes.Additionally, an examination of different lags (possible lags in quote reporting) do not change the findingsregarding the locked and crossed BBOs. See section VI for more details on locked and crossed markets.8

Effective Half-Spread i,t I i ,t (Pi ,t M i ,t ) , whereI it is an indicator variable that equals one for the customer-initiated buys and negativeone for the customer-initiated sells,Pit is the trade price, andM it is the midpoint of the NBBO quotes in effect for stock i at time t .11The effective spread, which is considered to be a better measure of executioncosts than the quoted spread, reflects the real price that market participants actually payfor immediate execution of their trades. The effective half-spread measures how closethe trade price comes to the quotation midpoint; which, in turn, is usually perceived as aproxy for the real value of the stock.In order to assess the “cream skimming” argument asserted by many previousmicrostructure studies (Bessembinder and Kaufmann (1997); Easley, Kiefer, and O’Hara(1996); and Battalio (1997)), we measure trades’ information content by assessing eachtrade’s price impact:Price Impact i,t I i ,t (M i ,t 10 M i ,t ) , where11Following Bessembinder (2003), trades are designed as customer buys and sells using the algorithmdescribed by Ellis, Michaely, and O’Hara (2000) instead of using the Lee and Ready (1991) algorithm.Bessembinder (2003) finds that although there is a difference between the results delivered by the twoalgorithms (results in case of using Lee and Ready are higher than if using Ellis et all), the overallinferences remains identical.9

M i ,t 10 is the midpoint of the NBBO quotes in effect ten minutes after the trade time.12A measure of trade execution cost that considers the possible effect of trades’differing price impact is the realized half-spread, defined for each trade as:Realized Half-Spread i,t Effective Half-Spread i,t – Price Impact i,tThe realized half-spread measures revenue to the liquidity supplier, net of the trade’sprice impact.Table II reports the effective half-spread, price impact, realized half-spread andpercent of orders which are price improved (panels A through D) for each of the marketcenters. The effective half-spread is the lowest for the electronic limit order bookmarkets and highest on the dealer/specialist markets. Specifically, the effective halfspread on the Cincinnati Stock Exchange averaged 0.71 cents, followed by ADF with0.84, the Pacific Stock Exchange with 0.93 cents. The effective half-spreads were higherfor the Chicago Stock Exchange with 1.04 cents, and then NASDAQ with a 1.10-centeffective half-spread.13 The results for AMEX are notably higher, 1.90 cents.Panel B of table II indicates that the price impact varies across the trading venuesfrom 0.25 to 0.46 cents (with a notable exception – that AMEX has a negative priceimpact). Consistent with the “cream skimming” hypothesis, the average price impact of12For trades executed in the last 10 minutes of the trading day, the closing quote midpoint is used. Werealize that the measured price impact for any trade can potentially contain a lot of noise due to the arrivalof new information. Nonetheless, it is assumed that averaging across a large number of trades is supposedto mitigate the noise and provide reliable estimates.13Using all BBO quotes (including locked and crossed market BBOs) does not change any of the resultsregarding trade execution costs, other than they are slightly smaller for each of the trading venues, but themagnitudes of the differences between the exchanges does not substantially change.10

trades completed on NASDAQ is in the middle of the other trading venues (0.31 cents)with only ADF and the Pacific Stock Exchange exceeding the NASDAQ result (0.42 and0.46 cents respectively). Unlike the other results in this paper, these results do not seemto indicate a reliable difference across market structures.According to the results from Table II, the average realized half-spread is thelowest on ADF (0.42 cents) followed by Island at 0.46 cents per share and Archipelago(0.47 cents). Again, the costs are higher for the specialist/dealer markets: the results forNASDAQ (0.79 cents) are not very different from those of the Chicago Stock Exchange(0.75 cents). The revenues to the liquidity suppliers on AMEX on the other hand, aremarkedly higher at 2.47 cents per share than the ones for NASDAQ, CSE, ADF, CHX,and the PSE.Table II also reports on the percentage of trades that receive price improvement,(i.e. trades executed at prices within the best quotes), as well as the percentage of tradesthat are executed at prices outside the quotes (panel E). Cincinnati provides substantialimpact to price improvement (54.27%), while the rates for NASDAQ, AMEX, ADF,CHX, and PSE are, correspondingly, 15.91%, 22.98%, 9.11%, and 10.67%. A sizeablenumber of trades are executed at prices outside the NBBO quotations, ranging from7.46% on NASDAQ to 34.50% on AMEX.IV.Time at QuotesIn order to measure the competition for prices between each of the trading venuesfor NASDAQ stocks, we examine the amount of time in which a trading venue is at theNBBO quote. Table III presents the results, with time weighted averages in panel A, and11

trade weighted averages in panel B. NASDAQ is at either the bid or ask 89.92% of thetime on a time weighted average and 86.52% of the time on a trade weighted basis.Interestingly, the electronic limit order book systems (Cincinnati Stock Exchange/Islandand the Pacific Stock Exchange/Archipelago) as well as the ADF are at either the bid orask more than half the time as well (which supports the findings of Huang (2002)).These results indicate a greater level of competition among trading venues than has beenseen for NYSE listed stocks, between the NYSE and regional stock exchanges asindicated in Blume and Goldstein (1997) and Bessembinder (2003).On a time weighted average (trade weighted average) NASDAQ, is at both theinside bid and ask the most often of the five trading venues, but they are alone only50.24% (50.31%) of the time.14 Two other markets (NASDAQ’s ADF and Archipelago,the Pacific Stock Exchanges) are at both the inside bid and ask frequent (34.18% and33.64% of the time), although Island is not.Table IV examines a percentage of a market volume conditional on its quotes.We examine each trading venues executed trades, and determine where their quotes arerelative to the NBBO at the time they executed a trade. While no real clear patternsemerge, it is evident that a trade will execute on a particular market regardless of howcompetitive the particular exchange is with their quotes.V.Trading Activity when not at the NBBOWhile table IV shows that market volume is not necessarily conditional onwhether an exchange is at the NBBO quote, table V (panel A) examines trading relative14The implication of this is that researchers using the TAQ database who are only using quotes identifiedwith the letter “T” are most likely over-estimating the costs of trading for NASDAQ stocks.12

to where the NBBO is. We find that NASDAQ gets very few trades (as a percentage)when they are not at the NBBO (2.93%), while AMEX and the Chicago Stock Exchangeget a substantial amount of volume when not at the NBBO (43.06 and 61.16%).Panel B and C of table V examines the number of times a market matches theNBBO and improves the NBBO. Interestingly, AMEX matches or betters the NBBO alarge percentage of the time (when compared to the other market venues). Additionally,the two ECN (Island (CSE), and Archipelago (PSE)), match and improve the NBBO issimilar percentages to those of NASDAQ. So, it appears that the ECNs provide similarquote matching and improvements as those that originate from NASDAQ.VI.Locked and Crossed MarketsSince the National Best Bid and Offer (NBBO) quotes are not provided by theTAQ database, we reconstruct it using the TAQ data. An unexpected result was thenumber of negative and zero NBBO spreads that are observed, and seem to be somewhatinconsistent with a logical perception of markets’ behavior.15 The Security TradersAssociation (STA) acknowledges that locked and crossed markets are an issue onNASDAQ. Currently they are proposing to bar access fees for exchanges and theirmembers who lock and cross NASDAQ stocks (Clary, 2003). Table VI presents thenumber and percentage of quotes that result in negative (crossed) and zero (locked)spreads. The average percentage of time the market is crossed (negative spreads) is2.24% or locked (zero spreads) is 12.43% during June 2003. The total percentage ofnegative and zero spreads during June 2003 is 14.67% of the NBBO quotes.15A discussion with Tim McCormick at NASDAQ confirmed that NASDAQ is having issues with lockedand crossed markets.13

Even though we find that the market is crossed or locked for our sample ofNASDAQ stocks over 10% of the time, there is no evidence that the markets are lockedfor extended periods of time.16 This is consistent with the statements by Chris Nagy,head of trading at Ameritrade “Though locked markets don’t last long, they occurfrequently”, (Schmerken, 2003).Table VII examines where the NBBO quotes are occurring for locked (panel A)and crossed (panel B) markets. As would be expected, locked and crossed markets donot occur with quotes from the same trading/quoting venue (there is not an instance inwhich both the ask and bid prices occur from the same exchange lock or cross themarket). Zero locked markets (zero spreads) occur most frequently when NASDAQ hasone side of the market and one of the other trading venues (other than the AMEX) has theother. A little different pattern emerges for crossed markets (negative spreads). Thisoccurs most often when the Chicago Stock Exchange has one side of the market andNASDAQ has the other. This is an indication that ECNs (in this case Island on the CSE)seems to be a detriment to the quote setting process.It is possible that the existence of non-positive spreads maybe attributable to theinterexchange miscommunication problems or TAQ reporting delays. Although, we didextensively examine whether the locked and crossed markets were a result of stale quotesor reporting delays, we find no evidence that stale quotes lead to the locked or crossed16An examination of BBO quotes, we find that markets will lock/cross, then unlock/uncross, and repeatthemselves periodically during the trading day.14

markets, and no systematic lag from any of the different market centers that wouldsubstantially change our findings regarding locked and crossed markets.17VII.Determinates of Trade RoutingPreviously we show that 48.41% of our sample trades are not executed onNASDAQ (NASDAQ captures 51.59% of the trades). Although quoting activity of thevarious market centers provides us with insights into the reasons for trade routing, we usea multiple regression specification to include additional variables into the analysis toexamine the determinants of trade routing.To determine the influences trade routing, we use a multinomial logisticregression specification for an unordered dependent variable, where the dependentvariable is the exchange where the trade occurred.18 Numerical values are assigned to thedependent variable as follows: 0 AMEX, 1 CSE, 2 ADF, 3 CHX, 4 PSE, 5 NASDAQ. A vector of regressors include the following: dummy variables capturingwhether an exchange is at the best bid or ask (6 variables for best bids and 6 for bestasks), relative order imbalance, relative number of trades in the preceding 10 minutes,relative volume, inverse of price, a dummy variable indicating if inter-exchange marketNBBO quote was crossed or locked, and the relative cumulative return. All regressors,except the dummies, are scaled to avoid a non-convergence problem often encountered17An examination of quotes which lead to the BBO being locked and crossed, there is no evidence that anyof those quotes were outstanding for as long as 5 minutes before they were updated (from initiation of thequote to the next quote update from the particular trading/quoting venue).18Trades were divided into customer buys and sells. Further (to facilitate the reporting of results), only theresults for customer buy orders are reported. Sell order results show similar results.15

when using iterative routines.19 We employ a fixed effects model by allowing forclustering across stocks and account for the possibility of non-spherical errors by usingthe Huber-White estimator.The following relations between the dependent variable and the specifiedregressors are expected. If an exchange posts an ask quote at the NBBO, a customer buyorder is more likely to be routed to that specific exchange. NBBO ask quotes for all otherexchanges should have a negative sign indicating competition among exchanges throughposting of the best quotes.Signs for NBBO bid quotes for customer buy orders could follow two hypotheses.On one hand, a bid quote should not affect the customer buy trade routing, so the relationbetween an exchange having a bid quote at NBBO and the probability of getting a tradeby that exchange should be zero. Another possibility of positive relations is a positivecorrelation between the decisions to post best bid and best ask quotes by market makers

ADF. In early 2003 the Pacific Stock Exchange began disseminating trades and quotes in NASDAQ stocks.4 The Archipelago Stock Exchange uses the Pacific Exchange as the market center in which it reports the trades and quotes