"There are two things that can happen here. One is a series may be tested for seasonality at monthly frequency and may not show any signs of seasonality," he said.
"What we found in our review is that in some cases those series did at quarterly frequency show significant seasonality, suggesting there is some seasonal factor that needs to be adjusted to at least a quarterly frequency."
Moulton said even if a series is adjusted at monthly frequency, which is the case with most of the source data for GDP, there may still be residual seasonality in some cases once the figures are combined to produce quarterly data.
The solution, he said, was to test all monthly GDP source data series at both monthly and quarterly frequencies.
Moulton said the BEA, part of the Department of Commerce, would work with other agencies to ensure that residual seasonality was removed from historical data by 2018. The BEA would also start publishing nonseasonally adjusted GDP and gross domestic income estimates in 2018.
"These estimates will allow users to isolate data revisions more distinctly from revisions to seasonal factors," he said.