One of the more interesting ideas I heard was by Andrew Lo. Physics is not a good analogy for the stock market. The laws of physics never change. Biology is better, since the market always evolves. I think fundamental indexing becoming popular may have hurt the screens. Or maybe just screening becoming so mainstream.

Another possibility is that our timescales have been too short to see that screens don't work all the time.
We intuitively know that. But, we don't know how long periods of underperformance might last.
If a screen "works" for 10 years, then doesn't work for four years, then works again for three years, is it broke?
I suppose it is, because few will use a screen that doesn't work for four years.

I recall one of the most famous example of a screen that stopped working is RS26, due to overtuning or herd effect.
It would be interesting to see if it still has not worked. I don't have VL, so am not familiar with it.

There are all these potential reasons why screens might not work ... fooled by randomness, herd effect, cycles, quants, machine learning and now AI.
It can also be combinations of these things.


Mark

Mechanical Investing FAQ

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