Re:Stock Market Prices Do Not Follow Random Walks
by FlorianOver on Tuesday February 09, 2016 @07:23AM
(#11064786)
> By analyzing the past returns of current S&P500 companies, the author is allowing for survivorship bias;
Thought so myself. Interesting to see what the result were with data from the beginning of the period.
Re:Stock Market Prices Do Not Follow Random Walks
by Flarbengaber on Tuesday February 09, 2016 @04:03AM
(#11064066)
I didn't quite understand your last paragraph, but it sounds interesting. Do you have a link?
Re:Stock Market Prices Do Not Follow Random Walks
by gjm11 on Tuesday February 09, 2016 @06:12AM
(#11064558)
Toy model: A fund can do well or badly at the start of the period, and it can do well or badly at the end. Both happen completely at random. A fund that does badly at both ends is closed and never heard from again.
If you analyse funds in this situation, you will find that every fund that does badly at the start of the period does well at the end. (Because the ones that do badly at the end too are all gone.) You might be tempted to think up clever explanations about how fund managers with bad initial results make extra effort, or how stocks that do badly tend to rebound later as investors recognize their true value, or something -- but that would be a mistake, because in this situation the only thing leading to the relationship between early and late performance is the fact that the "bad at both ends" funds aren't represented in the analysis.
Re:Stock Market Prices Do Not Follow Random Walks
by amazing_jose on Tuesday February 09, 2016 @05:48AM
(#11064482)
I think he means that lucky strikes can be longer than the career span of many managers. You never see them losing because they don't live long enough to have a devastating return to the average.
This is the same problem that a study ran into some time ago when it demonstrated that the portfolio managers with the worst returns were the best investments; it analysed the returns of a number of managers over a period and found that the ones with the worst returns at the beginning had the best returns at the end. The problem is that all the consistently mediocre or bad managers were discarded, as they did not survive until the end of the period.