Thanks for this feedback-- It's very accurate that 13Fs do not pick up hedge fund activity such as short sales and derivatives. 13Fs are not necessarily comprehensive of all of a single hedge fund's positions as certain funds sometimes make amendments at a much later date.

Hedge funds, particularly activist funds, do sometimes try to conceal the stocks they buy - usually by using derivatives which have different disclosure requirements. However, it is pretty rare for a hedge fund to be long and short the same stock unless they are doing it for tax reasons - this does not happen often. Going long a stock to create confusion about a short position (which is not disclosed) does not happen in the wild.

We believe that "knowing" in the aggregate just the stocks that managers are long has value. It is noteworthy that equity long/short strategies take a few quarters to materialize so using quarterly data is still valuable (hence we have excluded hedge funds that follow a short term statistical arbitrage strategy with many positions and very high turnover).