Chicago fund researcher Morningstar said last week that it will change its widely used formula for evaluating mutual funds. The changes, which will apply to its "style box" system, are slated to go into effect by mid-year.

Morningstar's style box, which evaluates whether a fund falls into value, growth or blend categories, is divided into nine squares that provide investors with a quick synopsis for how managers run an individual fund. The boxes' vertical axis notes small-, mid- and large-cap styles and the horizontal axis notes value, blend and growth funds.

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