Chicago fund researcher Morningstar said this 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 boxes, which have historically evaluated whether a fund falls into value, growth and blend categories, are 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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