Vincent Barabba, who led the bureau between 1973 and 1976 and 1979 to 1981, Kenneth Prewitt ( 1998-2001 ), Robert Groves ( 2009-2012 ) and John Thompson ( 2013-2017 ) said in a statement that the pandemic had delayed the start of many census field operations and also put back non-response follow-ups (NRFUs).

The statement said that the NRFU operations were originally due to take place between May and July 2020, but had been forced back and will instead start on 11th August and run through to the end of September.

Staff at the Census Bureau therefore decided a four-month extension  for the legally-mandated deadlines for dealing with the census data to April 2021 was necessary, according to the statement.

The US census is mostly carried out through fieldwork, which has been heavily impacted by Covid-19 restrictions in many parts of the country.

On Monday, the Census Bureau said it would hire more staff to try to meet its deadline of the end of 2020, and brought forward the end of fieldwork from 30th October to 30th September to allow more time to process the data collected.

The four former directors called on Congress to share the burden of deciding whether the data collected in the census was “substandard” and appoint an independent and apolitical body to examine the census.

The directors also said the Census Bureau should implement transparency measures, including daily reports on the completion percentage of NRFUs.

“The Census Bureau will not be able to carry out the NRFU fully and will be forced to take steps such as fewer in-person visits and rely instead on the use of administrative records or statistical techniques on a much larger scale than in previous census,” the statement said.

“The end result will be under-representation of those persons that NRFU was expected to reach and, at even greater rates for traditionally hard-to-count populations and over-representation of all other populations with potentially extreme differential undercounts.”