U.S. “Total market” indexes tend to be similar. However, when index providers carve up the total market, they can use very different methodologies, resulting in differentiated exposures for similarly labeled categories.

Even within index fund categories, each provider has its own distinct approach to benchmark construction. Methodological decisions determined by each benchmark provider—such as size segmentation, growth or value classification, and rebalancing cadences—can significantly affect portfolio composition.

Consider these distinctions:

• Size segmentation: There is no single standard among index providers. Some providers segment size based on number of constituents, while others do so on cumulative…