Most passive index-based exchange traded funds charge lower expense ratios that are comparable to mutual fund institutional share class fees.

On an asset-weighted basis, three of 14 ETF categories showed a lower asset-weighted expense ratio than their mutual fund counterparts, writes Alex Bryan for Morningstar.

Specifically, large-cap blend, mid-cap growth and small-cap blend categories whoed that ETFs have a lower asset-weighted expense ratio than mutual funds. However, when comparing the asset-weighted expense ratio, cheaper institutional share classes are also factored in.

“Index mutual funds’ institutional share classes pull the asset-weighted average expense ratio down,” Bryan said.

However, these institutional shares are only available through a high minimum investment or on select retirement platforms.

Consequently, on an asset-weighted basis, ETFs don’t have a cost advantage to retail mutual fund share classes when it comes to emerging market, foreign large-cap blend and small-cap value stocks. Compared to institutional shares, ETFs only have a cost advantage in the mid-cap growth category.