Lately, I’ve heard a few references to Dalbar’s measure of how much retail investors underperform the investments they hold due to poor behaviour.  I suspect that if the people making these references understood how Dalbar calculates this measure, they’d be embarrassed at having mentioned it.  There can be legitimate academic debate about the best way to measure investor underperformance, but Dalbar’s simple method is just nonsense. A simple example to illustrate the problem Ann has invested in ABC fund for the past 5 years.  Her initial investment was $10,000.  Over the first 4 years, she left her investment alone and it grew 50% to $15,000.  Ann then got an inheritance of $20,000, which she put into ABC fund to give her a total of $35,000.  In the final year, ABC went up 6%.  Ann now has $37,100. By any reasonable method of analyzing Ann’s investment behaviour, she exactly matched the performance of her fund.  She was always fully invested with ...