Brand valuations are bullshit, according to Mark Ritson, associate professor at the Melbourne Business School. And he told an audience at the Festival of Marketing just why this was so, outlining three "sins" committed by those whose business it is to come up with them.

First up was variation: how is it possible, he asked, that they can value the same brands so differently – not just a few millions either way but billions of dollars. There were "outrageous differences", he declared, offering up the examples of Apple and Visa. While the three leading brand valuation businesses – Interbrand, BrandZ and Brand Finance – all agreed that Apple was the world's biggest brand, Brand Z's valuation of $247bn was almost twice that Brand Finance's $128bn. The difference of $119bn was, said an incredulous Ritson, equivalent to the GDP of Belarus. And when it came to Visa, BrandZ's valuation was 15 times greater than that of Interbrand.

He moved on to his second sin of denunciation, arguing that the valuation businesses were reluctant to discuss the reasons behind these huge differences. OK, valuation may in some regards be subjective, he said, but "that's a shedload of subjectivities". He didn't feel that different methodologies and alternative dates of an assessment could answer that charge.

Discussing the third sin of deviation, he cited the work of trademark firm Markables, which looked at how brands have been valued when bought in a public deal and what the comparable valuations at the time were. So when HP bought Compaq in 2002, for example, the deal valued the Compaq brand at £1.4bn; Interbrand put it at $9.8bn. He suggested that one might expect a variation of plus or minus 20%, but over a range of examples none of the three had come close. BrandZ was the best at 98% (based on 13 observations), followed by Interbrand on 196% (22 observations) while Brand Finance came in at a whopping 323% (127 observations).

"I love the theory of brand valuation", Ritson concluded, "but in practice it's not fit for purpose."

Ranged against him in this debate were representatives of the same three businesses he'd just taken to task.  Michael Rocha of Interbrand pointed out that marketers create value in brands and create value for customers and it's useful to quantify the results. Putting a figure on a brand creates an understanding of, and language for, brands in the boardroom, he maintained, and helps grow both brands and businesses by enabling marketers to systematically measure and manage the levers of value creation. He added the caveat that "you need to find a model you believe in and sick with it".

Doreen Wang of BrandZ demanded to know what the problem was. "It's good to have three different valuations," she declared. "Why do we have to have one methodology?" She pointed out that the BrandZ methodology involved interviewing 3 million people around the world about their views on brands and "that's valued by clients". Further, she argued, brand valuation measures everything marketers are doing and connects marketing with business, increasing its share of voice in the boardroom. "It proves that marketing is not a cost but an investment to secure financial success in the long-term."

For Brand Finance, David Haigh said that brands can be valued like any other asset and highlighted ISO 10668, the global standard on the subject. The process was not bullshit, he said, even if the conclusions sometimes were. And, addressing one of Ritson's criticisms, he said that it was perfectly possible for valuations to range widely depending on the assumptions one used. People might have very different views on the impact Apple could have if it got into the TV market, he suggested, or on whether it would kill the watch market. Look at equity analysts, he added – they rarely agree and no-one complains that one values Apple shares at $200 while another puts them at $1,000.

Haigh also suggested that the Markables research quoted by Ritson was flawed, as finance directors regularly undervalued brands for tax reasons, something even the FRC has complained about. Cheekily, he cited WPP's takeover of Millward Brown, which manages the BrandZ valuations, and said the figures showed that there was no brand value in Millward Brown when it was acquired.

So bullshit or brilliant? Three against one might not be regarded as a fair fight but Ritson held his own on the day: those present voted 52% bullshit, 48% brilliant – and, allowing for a margin of error, that can be declared a draw.