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Comment by andylei | Hacker News Reader

http://papers.ssrn.com/sol3/papers.cfm?abstract_id=1100635

"Algorithmic trading has sharply increased over the past decade. Does it improve market quality, and should it be encouraged? We provide the first analysis of this question. The NYSE automated quote dissemination in 2003, and we use this change in market structure that increases algorithmic trading as an exogenous instrument to measure the causal effect of algorithmic trading on liquidity. For large stocks in particular, algorithmic trading narrows spreads, reduces adverse selection, and reduces trade-related price discovery. The findings indicate that algorithmic trading improves liquidity and enhances the informativeness of quotes. "

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Can you explain how this "improved" liquidity benefits actual people? It seems to be an assumption that any increase in liquidity is always beneficial. I've never seen an actual explanation arguing how.

To put it another way, how do we know the function of liquidity to net human benefit is monotonic (always increasing) rather than having a local maximum?

> For large stocks in particular, algorithmic trading narrows spreads

lower spreads means you pay less when you trade, simple.

Basically, algorithmic traders are disrupting what traditional traders used to do. In the future, Wall Street will be replaced with a couple of big server farms, and a Demand-Media style bidding system for commoditized stock analysts.

Imagine it - "Earn cash in your spare time - estimate the future profits of Fortune 500 companies, and plug them into a structured database! Senior members represent us on boards of directors, telling CEOs what the computer thinks they are really worth! (payments contingent on the long-term value of your work)."

Forgive my ignorance, but how so? How does the wealth extracted from the transaction play into this (arbitrage extracts value from a transaction, ideally it's made up for through liquidity).

What is improving liquidity? It greatly depends on how you look at it. Do more purchases provide more liquidity? Absolutely. If I previously had 10 buyers and now I have 100, I am absolutely more able to liquify.

However, to suggest that the entire 100 are needed to liquify when 10 will do (less than 30 seconds later I might add) is farcical.

actually, if you read the next sentence, you'd see how they measure liquidity:

> For large stocks in particular, algorithmic trading narrows spreads, reduces adverse selection, and reduces trade-related price discovery.

its not just a question of who will buy, its a question of who will buy at what price. if you have more liquidity suppliers, spreads will thin and you'll pay less.

That doesn't really show how they measure liquidity, but even if it did, I'm not particularly interested in how the framers of the article you linked measure liquidity.

I freely admit that if you want to measure liquidity as the amount of buyers in the marketplace, then HFT definitely increases liquidity. However, to suggest that HFT is the only source of liquidity, as I've said twice before, is farcical.

Also, it is totally accurate that HFTers reduce spread in the marketplace. They also increase the spread when they are driving the price of a stock up. Also, HFTers aren't unique to affecting the spread in the market. Every large scale buyer can change the spread.

Not only that, but my original point regarding the provision of liquidity and HFTers claiming they provide the essential job of supplying it (ie: without HFT, there would be no buyers) and I used the short period of time of the average stock position holding to suggest that the liquidity exists regardless of the existence of HFT.

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