ERG is part of the Cabinet Office and was set up by the coalition after 2010 election with the aim of working with government to cut costs and address wasteful spend.
Of the billions of pounds saved report, it was claimed that the digital transformation contributed £200m.
However, the NAO claims that methodologies and processes used by ERG are sometimes lacking accuracy, although it recognises the Department has done work to improve errors.
The organisation claims of the 14 areas of savings identified, nine had strong methods and evidence, while the remaining five were less strong – although, this doesn’t necessarily mean good savings weren’t achieved.
The Government Digital Service (GDS) was one Whitehall organisation that was identified as having weaker processes which led to errors being identified and removed.
GDS was also one of three areas including construction and major projects that needed improvement in 2012-13 and have continued to do so in 2013-14.
“The ERG has worked to improve the calculation of savings but there are still areas where the quality of the evidence must be improved,” claimed Amyas Morse, NAO head.
“ERG has undoubtedly achieved significant savings for the taxpayer, as well as general injecting pace and priority into the efficiency agenda.
“It should do more to strengthen its own processes. It should also look to improve its reporting on how the scope of the savings has changed over time,” Morse added.