The GIGO principle is a useful idea from the world of mathematics and computing. It stands for Garbage In, Garbage Out, and it means that a machine (or a mathematical process) will only give you useful results if you give it the correct data. You cannot, in other words, get good results from bad data. A calculator or an Excel spreadsheet will perform the calculations perfectly, but if you’ve mistyped the figures, the results will be meaningless. As my dear old nan used to say, you cannot make a silk purse out of a sow’s ear.