Adverts claiming that shoppers can save by switching to Aldi have been banned for a second time after Tesco complained that the discounter's price comparisons were flawed.
The television and press adverts for Aldi's 'Swap & Save' campaign featured four shoppers talking about their experience of swapping from their usual supermarket to Aldi, with one saying: "We've saved so much on our weekly shop."
On-screen text said 84 out of 98 people saved between September 16 and December 1 last year after shopping at their usual supermarket for four weeks and then Aldi for the same period.
It noted that non-grocery and high-ticket items which did not form part of a typical weekly shop were excluded.
But Tesco complained that the comparison was misleading because it believed the eight-week comparison period was out of date and invalid for a price sensitive market, the weekly shops were not compared on a like-for-like basis, the selection of items to include or exclude was arbitrary and explanatory information in the adverts was not sufficiently prominent.
Tesco noted that the adverts stated that 84 out of 98 people saved, and that the challenge was based on four weeks' shopping at a competitor and then four weeks shopping at Aldi, but it understood that only four of the 98 individuals had undertaken the eight-week challenge and the remaining 94 had undertaken the challenge over two weeks.
Aldi said Swap & Save differed from more traditional comparative advertising as it looked at the overall shopping costs at a single store over an extended time rather than the prices of individual products or a standardised basket.
It said comparing individual products would therefore have been artificial because consumers did not buy the same items every week.
It said the campaign fully compared the actual overall weekly shop of individuals over a lengthy period and was fair, transparent and verifiable.
The ASA said it was acceptable for Aldi to run a price comparison campaign based on consumers shopping normally with a competitor for four weeks and then with itself for the same time.
But it believed consumers would assume that 98 people had participated in the eight-week challenge, when in fact only four people had.
It also found there were inconsistencies in how items had been included or excluded, with some non-grocery or high-ticket items such as dishwasher salt, truffles and Christmas pudding excluded from one shop and included in another.
The ASA said its own calculations, which took into account items it believed should have been included and excluded and allowed for inconsistencies and inaccuracies in Aldi's interpretation of data, still showed that savings in the original shops ranged from 22% to 33% and savings for a shorter "revalidation" trial ranged between 25% and 38%.
It concluded: "We considered those amounts represented significant savings at the time of both the original shops and the revalidation shop, and therefore that the overall message of the Swap & Save campaign, that consumers could save money by shopping at Aldi, was not misleading to consumers.
"Nonetheless, because we considered the way in which the comparison was presented in the ads implied more people had participated in the eight-week challenge than was the case, we concluded the ads were in breach of the Codes."
It ruled that the adverts must not appear again in their current form, adding: "We told Aldi Stores Ltd to ensure that in future the basis for comparisons was made clear and did not mislead, and that their comparisons were verifiable."
Aldi's joint managing director of corporate buying, Giles Hurley, said: "It is important to be clear that the ASA has endorsed the validity of the Swap & Save campaign.
"We recognise that the decision to uphold competitor representations was technical in its nature and, because of this, we will respect the finding and have addressed the issues that it raised."
Earlier this year, the ASA banned a TV advert for Swap & Save following a complaint from Asda, warning Aldi to make the basis of its comparisons clear in future.
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