This AQR working paper argues that the size effect does, in fact, exist. The crucial element of their approach is to control for exposure to other factors when estimating the size premium, in particular regarding high quality versus low quality (junk) stocks.
Using this approach, the authors conclude that a significant size premium emerges, which is stable through time, robust to the specification, more consistent across seasons and markets, not concentrated in microcaps, robust to non-price based measures of size, and not captured by an illiquidity premium.
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