Portfolio managers can’t pick stocks – this is a common saying in the popular press and between proponents of index investments. But research shows it is not such an evident truth. Mutual/hedge fund managers, in reality, can pick stocks but are often too diversified, and their “best picks” therefore cannot deliver such a spectacular performance as the fund’s performance is dragged down by the rest of the portfolio.
The 13F Fillings Following system is based on the assumption that stocks in which mutual fund managers (and hedge fund managers) are mostly concentrated (their best ideas) are stocks that will outperform the broad equity index. SEC 13F fillings could be used to track top holdings positions in mutual funds.

Mutual fund managers must have highly diversified investment portfolios as current investment doctrine doesn’t recommend highly concentrated portfolios, and most investment managers have fear to divert significantly from the relevant benchmark.
But stocks which are managers’ “best ideas” (high-conviction positions) are often over-weighted in their portfolios. These are also usually stocks which are most understood by those managers. It is, therefore, fundamentally feasible to track those “best ideas”.