Be warned: your friendly local barman may be serving you short. Nick Barber calls time
Never mind whether the glass is half-full or half-empty. A more pertinent question is whether a pint glass is completely full, or only 95 per cent.

According to a survey published last week by the Campaign For Real Ale, eight out of every 10 pints served in British pubs are below the "95 per cent liquid" level recommended by the industry. It's only a few millimetres per glass, but it deprives drinkers of pounds 1 million-worth of alcohol each day.

The reason for these short measures is not, of course, an altruistic desire to reduce any damage to our liver. Pub staff are far more likely to be thinking of their "stock bonus", says "Dave", a Glaswegian barman. (To ensure Dave's safety, his real name has not been revealed, neither has the name of the Irish pub where he works.) "The landlord weighs all the bottles and kegs every week to work out how much drink you've sold," he reveals. "They compare that with what's on your till roll and they work out whether you're 'up' or 'down' on a particular beverage. If you have more drink left over than you should have, you're up, and you get a stock bonus. And the only way to get up is by short-serving people."

Lager is the most lucrative drink, because of its barman-friendly foam- to-beer ratio. "Guinness is such a straight pourer that it is difficult to give it much of a head," confides Dave, "but with heavy and lager, it is easy to fill them up with froth."

The impoverished barman's second favourite tipple is any spirit that comes with ice or a mixer. "At our pub we don't have optics, we have wee metal measuring jugs, so we can fill them up to whatever level we want, and distort it with the mixer. If people bug me, I put in half a measure and top it up with ice."

Are there no depths to which these cheating bar stewards won't stoop? Apparently not. "I remember this guy came in and bought a pint of lager," says Dave. "He only stayed for about five minutes and he hardly touched it, so when he left he brought it back and said, 'Pour this down the sink.' I thought, 'Hmm, no. I'll hang onto this.' And when someone came in and ordered a pint of lager a few minutes later, I just topped up the first one and served him that. I basically sold the same pint twice."

Nigel Griffiths, the Labour Party's consumer affairs spokesman, has promised that beer glasses will be bigger under Blair. Meanwhile, what's a drinker to do?

"Short of taking a measuring stick to the pub," says Dave, "all I could advise is to ask the barman to top up your pint if you think you've been short-served. Or else, look into their eyes while they're serving you, and if they look guilty, chin 'em."