I have to admit I've only just started learning to trade/invest myself. I'm quoting books and courses I've been working through. It is a sweeping statement, but there seems to be a lot of evidence that supports the Efficient Market hypothesis (the idea that all information about a securities price is already 'priced in'). Any new information is rapidly priced in long before any retail investor can exploit it, ie, almost immediately.
HFT's and algos are perhaps proof of this, ie, how you have to heavily invest in equipment and expertise just to exploit a tiny arbitrage opportunity unrealised before. This is an opportunity created by an advancement in technology rather than a refutation of EMH. If not why not just hire analysts and save a lot of money?
Not sure about indexes but passive investing and the actions of the herd could drive prices up unrelated to the fundamentals and you get an overpriced market and consequently a bubble? (This is a strategy I may follow)
I think Kendall was showing that price changes are independent of one another and therefore unpredictable.
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Below is a chart that shows potential returns from day trading.
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