(Bloomberg) -- Trying to time the market is a dumb idea.

That in a nutshell is Cliff Asness's opening salvo into an escalating war of words over smart-beta exchange-traded funds, triggered in February when one of the inventors of the approach said many had become too expensive. Asness, co-founder of AQR Capital Management, joins other theoreticians jumping in the fray, which goes to the heart of a strategy that bridges active and passive management.

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