It’s hard to unite nationalisation advocates and sceptics. But the Confederation of British Industry has managed to do just that, getting into hot water this week with an unsubstantiated claim that the Labour party’s plans to renationalise utilities would cost £196bn. These kinds of big numbers get media attention and help drive a narrative, particularly when they come from an establishment organisation such as the CBI. But by presenting an unbalanced view it has lost out on an opportunity for genuinely useful public debate.

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The organisation, which lobbies on behalf of British business, has already been forced by Labour to admit its mistake. The analysis assumed that a Labour government would purchase the rail industry’s rolling stock – trains that are privately held and leased to rail operating companies – even though this is not party policy. While this mistake is important, as it has misled the public, its effect on the overall figure is likely to be relatively small.

There are far bigger problems with the CBI’s analysis. For example, in calculating how much the government would pay for the companies operating in the industries it plans to nationalise, the CBI applied a 30% mark-up to the firms’ asset values. This is based on “evidence of historical takeovers which shows that utility companies have typically fetched a price above the asset value”. Yet this higher price reflects that, as essential services, they effectively enjoy a guarantee from government, as well as the value of economic rents that can be generated in monopolistic industries. This would not be a fair price. There is no reason why the government would pay for its own guarantee – nor should it pay for the rent extraction that nationalisation seeks to stop.

Even more fundamentally, the analysis only assesses the costs of nationalisation, rather than the potential benefits. It does not take into account income that would be generated by the companies brought back into public ownership – from ticket sales or energy and water bills, for instance. While the CBI was happy to reference future costs, including maintenance, running costs and investments, it hadn’t considered the asset side of the balance sheet.

That’s a shame, because a genuine debate on public and private ownership would be welcome. There are legitimate questions about which industries and companies should be publicly owned, which ones should be in private hands and indeed whether alternative models such as community or cooperative ownership would deliver better outcomes. Those questions should consider not just the cost to government but also who benefits and loses in society from different ownership models – both financially and in terms of who has control over the economy. They should consider who will best manage services that we all rely on and ensure sufficient investment for the future. They should also consider assets as well as liabilities.

Facebook Twitter Pinterest ‘The business membership organisation, which lobbies on behalf of British business, has already been forced by Labour to admit its mistake.’ Photograph: Toby Melville/Reuters

It is not just the CBI that fails to do this. The UK government’s “public sector net debt” target fails to count many income-generating assets that could pay for themselves. By including public corporations in this calculus, a large bias against public ownership is built in. If Germany had taken the same approach, in 2015 its general government gross debt would have been more than 2.5 times higher, at 181% of GDP. Thankfully, our national accounting rules are increasingly being called into question, with suggestions that the government should instead target public sector net worth to assess both sides of the balance sheet – as any private investor would.

The CBI is upfront about omitting the potential benefits of nationalisation. But if it wishes to be considered a trustworthy and fair voice in our public debate, that simply isn’t good enough. Voters cannot be expected to dig into the details and check how the big figures are calculated. Even if they wanted to, they would not be able to fully assess the CBI’s claims. Worryingly, the group has said its members “do not feel comfortable” explaining how they got to the total of £196bn.

As the parties gear up for a potential general election, there will be many more claims and counter-claims to come. The business lobby group’s eagerness to produce news splashes on Labour’s policy plans suggests it believes it could be a close outcome. But rather than publishing partial analyses, the CBI should focus on the real questions at hand. With trust in business reaching record lows in the past few years, interventions such as this will do little for the strained relationship between it and the rest of society.

• Carys Roberts is chief economist and head of the Centre for Economic Justice at the Institute for Public Policy Research