How reliable are official statistics?

According to the Scottish Government Food Waste campaign, if we used up all the food and drink we waste, it would cut carbon emissions by the same amount as taking one in four cars off Scotland's roads. The same Greener Scotland campaign claimed the saving to each Scottish household could be £470 a year.

That is a specific figure. How can the statisticians know what the average household throws out when different people have markedly different diets? How do they calculate carbon emissions arising from it? And how can they know so specifically how much money people are wasting?

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The figures come from the report Scottish Household Food and Drink Waste in 2012 by Zero Waste Scotland. It looked at how much household waste collected by councils consisted of food. (One caveat: it also considered English and Welsh data.) In addition to this, it then estimated how much food is wasted that never makes it to the rubbish bin, because it gets put on the compost heap, poured away (such as fizzy drinks that have gone flat) or fed to the dog. It did this by asking a number of households to keep food waste diaries. All that information was then scaled up to the number of households in Scotland in 2012.

Bang up-to-date fieldwork on exactly what types of food and drink types make up this waste in Scotland does not exist, though new data should be available soon, so for this analysis the data comes from the study Household Food and Drink Waste in the UK 2012.

The cost of each type of food waste is calculated from UK government statistics looking at the weight and price of particular food types bought by households. This allows for an average price to be applied to each foodstuff, taking into account different prices and the share of the market for different products (in other words that some cheese will be premium range and some will be from cheaper ranges). The prices are 2012 prices though, since there has been price inflation since, it is likely to be an underestimate.

Putting food in landfill is so bad for the environment as food rots to produces methane, 25 times more powerful a greenhouse gas than carbon dioxide. Yet we also have to take account of emissions generated in growing, harvesting, processing, transporting and preparing that wasted food.

Dave Reay, reader in carbon management at Edinburgh University, explains that for a litre of milk, 1kg of greenhouse gas emissions are generated on the farm producing it, given that cows produce an awful lot of methane.

So if we pour away a litre or milk, it has to be replaced by more milk, causing more emissions. Ultimately, if we didn't waste food, we would need to produce considerably less of it and use considerably less land. In fact, food waste accounts for about 10% of our greenhouse gas emissions worldwide, says Mr Reay.

I digress. To come back to those campaign statistics, they are estimates based on an amalgam of Scottish and, where necessary, UK data. They are robust estimates with careful methodology behind them. Even if that £470 figure is more of a guide than a promise, it is clear we are in the habit of pouring money down the drain.