In the weekday-effect discussion, Niederhoffer distinguishes reported differences from money-making opportunities. The selected passage explicitly compares the size of effects with commissions and bid-ask differences and notes instability across periods.
Use two separate questions
Ask first what the data support, then whether the supported effect would remain useful under realistic assumptions. This companion has not run a strategy test. A clear distinction between these questions helps avoid overstating either the book’s results or your own.
Worked example
A hypothetical average effect of 0.05% may be detectable in a very large dataset. If a completed trade costs 0.12%, it is not a positive net opportunity under those assumptions.
Case connection
A useful comparison concerns net outcomes, not merely an impressive statistical claim. Fees belong in the economic question.
Buffett’s ten-year fund wager
Source-grounded facts
Buffett’s 2017 letter reports that the S&P 500 index fund beat each of five funds-of-funds over the wager.
Context
Buffett and Protégé Partners compared an S&P 500 index fund with five funds-of-funds over ten years, from 2008 through 2017. The comparison included the returns investors retained after fees.
Outcome
One comparison fund was liquidated in 2017, a fact noted in the table. The wager illustrates costs and evaluation periods, not a universal result for every manager or decade.
- All five funds-of-funds outperformed the index fund in the difficult first year, 2008.
- The comparison continued through the agreed decade rather than stopping after that initial result.
- Buffett’s final table reported a 125.8% gain for the index fund; none of the five funds-of-funds matched it.
Case analysis
A statistical difference and an economically useful difference are distinct claims. Costs can remove a small gross advantage even when the difference is consistently measured. Keep the evidence for detection separate from the assumptions about execution and money retained. This comparison illustrates the economic question; it does not establish statistical significance for a new trading rule.
Try it
Write a one-sentence statistical claim and a separate one-sentence economic claim for the same fictional result.