The Securities and Exchange Commission investigation focuses on assumptions the six use to calculate current pension expenses, which can affect reported income.

Under standard accounting practices, companies don't use the actual returns on their pension assets to calculate costs, because this could lead to large swings in results from year to year. Rather, they use "expected returns." In recent years there has been growing criticism that companies are using artificially high estimated rates of returns on pension assets to lower their current pension costs.

The "cost" is the amount that gets included in quarterly calculations that show the impact of the pension plan on the company's income.

The identities of the six companies were not disclosed.