Are virtual shopping baskets bigger than real ones?

How much did you spend on those groceries you bought online from Tesco a couple of months back?

£50? £70? Can't remember? We need an answer now, so give it your best shot.

It may seem trivial, but the holes of memory that are shown up in a Q&A like this – a daily problem for market researchers – go to the very heart of one of the stories the news sites and blogs alike got most excited by this week: how much we shop on the internet.

On Monday, IMRG, the body which represents internet retailers, announced that online shopping had risen to an all-time high in July of £4.2 billion – up from £2.34 billion in the same month a year ago.

This, the IMRG said, could be attributed to the poor weather, and the fact that more and more retailers had better websites enticing us to 'add to basket' and 'proceed to checkout'.

As a teaser in its press release, the IMRG casually dropped in that high street sales in the same period had only totalled £5 billion - the implication being that online might be en route to surpass the high street.

Many papers reported the story with relish. 'Rainy July pushes online sales past £4 billion' trumpeted the Guardian. 'Online sales set record in July' said the BBC.

Was the comparison fair, though? And to what extent could we be assured IMRG's figures were accurate? After all, eAbsinthe.com apparently does a bright trade in specially slotted Absinthe serving spoons in the UK. Had they been included?

The first bells started to ring during Times Online's research for the story when the Office for National Statistics (ONS) – to which the £5 billion figure was attributed – said that it included 'online' in its 'high street' sales in any case.

"There is a category called 'non-store retailing and repair', which would cover specialist internet retailers," said a spokesman for the ONS, which bases its figures on actual data from about 5,000 stores. "In 'food', for instance, Tesco would give us both sales for in-store and online. There's no way of separating."

The IMRG's £4.2 billion for online alone suddenly look all the more ambitious. How did it calculate its figures?

"We base our figures on data from 80 online retailers, which account for about 20 per cent of the total market, so you can extrapolate out," James Roper, chief executive of the IMRG, said.

And how do you know they account for the 20 per cent?

"Based on figures from previous years."

But where do you get the multiple from? Surely at some point, you have to know what the total is?"

"The total is an estimate based on a forecast," Mr Roper went on.

Blackadder's immortal words to his sidekick Baldrick – "I think I'm on the point of spotting the flaw in this plan" – crept into the silence at the end of the line.

Whose forecast?

"Forrester's," he said.

Forrester is a highly respected analyst firm which has been researching and compiling figures on the online retail market for seven years.

The particular forecast relied upon by the IMRG, Times Online discovered, was reported in June last year in a document titled Forrester Research European eCommerce Forecast: 2006 to 2011.

When contacted, Jap Favier, the chief analyst on the report, said that Forrester's figures were based on data from "both the supply and demand sides."

"We routinely ask consumers: do they buy online? How much do they spend? When did they spend it," Mr Favier said, and from the total spent by an average UK internet shopper, a total for the whole of the country is extrapolated.

Does that not depend a lot on what people remember they spent, rather than what they actually spent?

"It does, and sometimes they're asked about purchases three months back, but I think it's pretty reliable. It evens out. Say one guy remembers the garden hoe at a DIY store cost him £10 when it was in fact £15. The next guy might remember is was £20, so with a sample size of over 1,000 people, it's not really an issue."

Maybe so. And with relatively new markets, where data collection is still in its infancy, some data is better than none.

But compared with the ONS's figures, which are based strictly on actual sales, IMRG's are a bit 'freer', to say the least.

Did the IMRG genuinely think online sales were set to take over those made on the high street?

"We don't think they'll surpass high street sales, no, but the role of shops will change," Mr Roper said. "Online will come in store. You might be in a store and know what you want, but they haven't got your size, in which case you could order it over the internet."

Which is another prediction altogether. But one, happily for the IMRG, that might spare it the task of separating out which shopping items which have been added to virtual baskets, and which to real.