Who's paying?

Also worth noting: Workers, of course, contribute to their 401k out of their own pocket, unlike the traditional pension plan, which is fully funded by the employer.

"There are different financing streams going on here, I admit that," VanDerhei said.

His focus was, he said, "from a retirement-income adequacy standpoint, which of these two [types of plans] seems to be providing larger retirement account balances?"

than he would have enjoyed with a pension plan, at the median.

He added: "The sole objective was to look at: 'Is Plan A or Plan B providing a larger source of retirement funds at age 65, not adjusting for how those are financed?"

In an ideal world, VanDerhei said, workers would have access to both a traditional pension and a 401k or similar defined-contribution plan.

"If you have both," he said, "then obviously you have less employee investment risk, you have less employee longevity risk and you still are giving employees the upside if they choose to participate in the 401k plan."

In his study, VanDerhei points to previous research that came to a similar conclusion: In many cases, retirement savers ended up with more money under a 401k than they did with a traditional pension plan. See the full EBRI report.

What was left out

The study is based on a robust sample of more than 2 million people. "I know what they're doing in their 401k plan. I know their salaries," VanDerhei said.

"Given the specific set of job changes and wage paths that I assumed for them under the 401k, I did the exactly same person, exactly same assumptions, and said: 'What would you end up with if you had a [traditional pension] plan?'" he said. "If you really do want to have a direct comparison of Plan A versus Plan B, I think it's helpful to actually track the same person under the two plans, and do the person by person comparison."

This study does not include workers who are automatically enrolled into their 401k by employers, though he said that would likely push the results even more in favor of such plans.

Also, the data looked at private-sector pension plans. It did not include public-sector plans. Doing so would have pushed the data more in favor of pensions.

"I can promise you I would get much different results had I done a public defined-benefit as opposed to a private defined-benefit comparison," VanDerhei said.

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Public pension plans -- those offered by government entities -- generally offer more generous benefits than private-sector plans, and usually include cost-of-living adjustments in retirement.

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