Predictability of Index Funds

A third advantage of index funds over active funds is their predictability. I am not talking about being able to predict with a glassball what the market is going to do next year. However, I can predict with great accuracy that an S&P 500 index fund will match the performace of the S&P 500 index. In addition, I can predict fairly well what stocks the S&P 500 index fund will hold next year. The reason I say fairly well is because the index changes very little each year, often due to mergers and acquisitions, which I cannot predict. Mutual Fund managers are sometimes known to change their investment style over time. In other words, your Large-Cap Growth mutual fund may slowly become a Mid-Cap/Large-Cap/Growth/Value mutual fund with or without you knowing ahead of time. This is called "Style Drift." Managers of index funds do not practice "Style Drift." That is why they are predictable.