Odds of Being Right on a Trade

On the topic of whether it makes sense to try to measure if you made a good stock investment either annually, monthly, or daily and what is random noise versus meaningful trends, there are a wide amount of opinions among us.

Most agree being 50/50 on a particular day is generally true, but over much longer periods of time the odds start to favor an increase. Next time you feel bad about making poor trades consider the fact that if you assume 50/50 chance of being right for a particular trade to make this simple, then the odds of having two consecutive good trades is 25%, 3 consecutive good trades is 12.5%, …

Try this exercise: For the next 15 business days (three weeks) try to predict where the markets are going to go the following day. To keep it simple, all you have to do is predict whether it will go up or down (you need not worry about how much). The chance of you getting all 15 days right is less than 1 in 33,000. To put this in perspective, you have a higher chance (1 in 9,000) that the Earth will be struck by a huge meteor during your lifetime.
above is from:


Many also falsely believe that after a run up for consecutive days increases the chances of the next day being down. The following author took 20 years of the S&P 500 market data from 1993 to 2013 and plotted the distribution of the market return the following day after 3 up days, after 4 up days, and after 5 up days. It basically showed a normal bell curve distribution around zero for the next day for each of the scenarios.


Bottom line: We should not be surprised in how often we are wrong in a trade. The math basically says it is guaranteed.


Great S&P Calculator that can adjust for inflation and for dividend returns over any period of years. The below table is adjusted for inflation and for including dividend reinvestments.

# of years	 Mean		Std Dev
    1		 9.65		  0
    5		13.52		 9.71
   10		 6.77		17.48
   20		 7.11		17.81
   50		 7.35		16.88

Using a Mean of 7 and the Std Dev of 17 one can then look at how a portfolio return might behave over time. If you use a bell curve distribution for your portfolio that means 68% of the time the return is within -10% and +24%. This means 32% of the time your returns are below -10% or above +24%.

Bottom line: This reinforces the need to hold thru drops and not sell. Let time fix your performance results assuming you have a diversified portfolio.

1 Like

assuming you have a diversified portfolio.

This is, of course, one of the gotchas. One needs to recognize if one is holding something which is going to get hurt more or more permanently than the market. E.g., in '08 I had a friend who was holding a rather large chunk of Bank of America, not because he had bought it thinking it was a great investment, but because he inherited it from his father and since then it had been doing OK. Not having his own personal investment thesis about the company, there was no trigger for him to get out and the result was that '08 took longer to recover from for him than many people. Now, the flaw here really was not dealing with the investment when he first inherited it, either to dump it and roll it into something he understood or to do the work so that he would know the company well enough to make choices about staying or leaving.

But, not everything recovers or recovers at the same rate.

1 Like