According to a poll conducted by the World Cancer Research Fund and reported by BBC News, just one in five Britons eats the recommended five portions of fruit and vegetables a day.

If we are to take this seriously, it would be useful to know whether we are missing the target by a little or a lot: if the average number of portions consumed is 4.5 then that is a lot less worrying than if it is 1.5 - particularly as the recommended figure of 5 is surely a somewhat arbitrary round number. And this is not the only respect in which the survey falls short.

It appears that the poll (conducted by YouGov) is based on people’s reports of their own behaviour: “the highest levels of consumption were reported in the south of England, where 26% said they ate at least five portions.” However, it is well known that people are often inaccurate in reporting their own behaviour, particularly if that behaviour might reflect poorly on them.

But in this case there is a further reason to think that the reported figure may be inaccurate: it really isn’t clear what constitutes a portion.

The story on the BBC website carries the following information.

WHAT IS ‘FIVE A DAY’?

• It equals around 400g of fruit and/or veg
• One portion is 80g
• A portion equals two or more small fruits ie satsumas, one medium-sized fruit ie an apple or banana, or half a grapefruit or one large slice of melon
• A portion of veg would be two broccoli spears or three heaped tablespoons of cooked vegetables such as carrots, peas or sweetcorn
• Potatoes do not count
• But fresh, frozen, tinned and dried fruit and vegetables do
• Smoothies can count for up to two of your five a day
• Pulses and beans count as one portion – no matter how much you eat

No doubt this is intended to be helpful, but:

“It equals around 400g of fruit and/or veg”: does this apply to everyone, independent of age, sex, body weight, activity level, metabolic rate?

“A portion equals two or more small fruits”: wouldn’t more than two small fruits be more than one portion?

“One medium sized fruit ie an apple …”: what is meant by ”medium’ here? a quick Google search suggests that apples range from about 100g to over 200g. (And ‘ie’ should be ‘eg’.)

“A portion of veg would be two broccoli spears”: are broccoli spears always 80g each?

“Three heaped tablespoons of …”: are all tablespoons the same size, does everyone heap them to the same extent?.

“Smoothies can count for up to two …”: what do ‘can’ and ‘up to’ mean here?

This isn’t just nit-picking. Vague, non-standard units and broad-brush averages really are unhelpful. Without clear, unambiguous and specific information a campaign such ‘five a day’ cannot be expected to succeed.

And if our diets do need improving for the sake of our health then that is serious.

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Neil Sheldon has taught at The Manchester Grammar School for 40 years. He is a Chartered Statistician and Fellow of the Royal Statistical Society. He has been an RSS Guy Lecturer since 2007.  He is also course leader for the Certificate in Teaching Statistics offered by the RSS Centre for Statistical Education

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