Getting data for such a study is hard; I know only the words of the paper on the study's conclusion of net. Size of market share determines spreads so it would have varied considerably based on the type of company. Other than reading the paper and its limits I don't know what to conclude. It would be interesting to have included Schwab since it purchased Ameritrade. Fidelity was decent.
If you can always get your trades executed at the midrange of the spread, good for you; the risk is missing trades when you don't. For longer term traders it shouldn't matter for big stocks; I trade short term so missing out on a trade altogether is more of a risk for me. E.g. I bought TPR a few months ago and it is up 62%: if I had missed it due to a misjudgment on my limit order I would have missed that gain.

Mechanical Investing FAQ

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