Confusing food prices?

I caught a story on the BBC the other day regarding confusing food prices - main thrust was that the "price per" section that retailers are obliged to provide may use difference units. As such food should be measured by litres (volume) or weight (kilograms) the reporter than showed how this would in no way be helpful by picking up two squeezy jars of mayonnaise and showing one marked by volume and the other by weight.


Sadly looking at how this story is being reported it's not being done very well. A fine example is This Is Money when they ask "which is cheaper?" a pack of 5 bananas for £1 or 68p per kilo. They go with the 68p per kilo... but it depends on how much the bananas weigh - they're variable. If each of the packed bananas weighed ~290g, 5 would weigh 1.47k and cost £1 at 68p/kilo.

The simplest rule should be that solids are sold by weight; fluids by volume. For those combinations such as pears in syrup go by percentages - more solid than liquid, kilograms; more liquid than solid; litres. Products can still be sold as a ml/cl/l or g/kg; but the 'price per' should always be the 'parent' measurement.

What this doesn't mean is that you can compare, say, a 300ml jar of mayonnaise to a 300ml jar of ketchup because they may well have different consistencies and thus different densities. it might be a really thin and watery ketchup with a density approaching 1 (weight/volume by definition water has a density of 1 in kg/l terms) whereas the really thick mayo may be 1.3.

By weight the ketchup may be 300g, but the mayo would be 390g. If the ketchup is 67p and the mayo is 80p which is the better value? By volume it's £2.23/litre vs. £2.66/litre. By weight it would still be £2.23/kg but vs. £2.05/kg.

But wait. What if you have a thick ketchup and a thin ketchup I've just shown a comparison by volume does nothing to help the customer. Similar to the mixed fluid/solid. If I put my pears in water it'll weigh less than if I put them in syrup; so I can use less pears to get equal weight.

Perhaps measure by quantity? Well if we go for the 'a banana costs 30p' route everyone will grab the largest bananas first because they'll be more banana for the price. Also wouldn't be any use with fluids unless it's marked per serving - so who gets to define "serving"?

So a modification of my rule. Solids by weight, liquids by weight AND volume. I can hear the outcry of 'Oh no not another thing to stick on the label and confuse shoppers' we're not stupid. If we see two comparable products and one has a higher price per weight but the other a higher price per volume we can work out the latter is denser than the former.