Leading article: Quick opinion polls do not reflect the real debate in this crucial election

The gap between perception and reality bedevils modern politics. That is particularly true during a general election in the information-rich world of instant news where the speed of communication is matched only by the shallowness of what is communicated. The outcome of the final election leadership debate this week was a case in point.

The televised leadership debates are a useful addition to British politics. They allow voters to form judgements about the main party leaders which are unfiltered by the lenses of our national newspapers, most of which present the news from a distinctive political slant. But such debates have their limitations. The restrictions placed upon the format have allowed the three leaders to get away with broad-brush statements which seasoned presenters like David Dimbleby must have been itching to subject to a far more detailed scrutiny.

The limitations of the format were underscored by snap opinion polls commissioned by the media in an attempt to force the outcome of the debates into a more newsworthy format. After the final debate the Tory press was enabled to pronounce that David Cameron had "won" by various margins or, at the very least, had drawn with Nick Clegg. But in all Gordon Brown was deemed to have "lost".

Those polls may presage the final outcome. But they reveal the hardening attitudes of the electorate rather than offering an objective verdict on the outcome of the debate itself. Such polls seem to mirror the attitudes viewers had before the debate actually began. With the Mrs Duffy affair the tide is seen to have turned from Brown, and the natural predisposition of individuals to want to back a winner has come into play.

The truth is that the Prime Minister acquitted himself rather well in the final debate. Messrs Cameron and Clegg may have scored on style and sympathy but Mr Brown was solid on substance. The Conservative leader repeatedly failed to answer, or indeed even to address, Mr Brown's criticism that it was unfair and immoral to benefit Britain's richest families by raising the threshold for inheritance tax while at the same time cutting child tax credits. Nor did Mr Cameron offer a convincing justification for his insistence that public spending is not playing a key role in stimulating the demand on which the tentative recovery of the economy depends. Cuts to public spending would jeopardise that – a judgement which the makers of economic policy in the US and throughout Europe all share. Mr Cameron merely dismissed Mr Brown's grave warnings as "desperate stuff from someone in a desperate state". But the truth is that the Conservative strategy here is highly risky.

Voters may not agree, much as they do not generally seem concerned at the gaping holes which the Institute for Financial Studies pointed out this week in the spending plans of all three main parties. They may be more influenced by melodramas like the Mrs Duffy gaffe, which one poll has suggested could knock 7 per cent off the Labour vote. But they should probe more deeply than the televised leadership knockabouts have permitted. Gaps between perception and reality are dangerous at any time, but no more so than in the final week of a general election campaign.