Dr. Max Hooper published his original formula in the book "Hedges" in 1974. This method is only a rule of thumb, and can be off by a couple of centuries; it should always be backed up by documentary evidence, if possible, and take into account other factors. Caveats include the fact that planted hedgerows, hedgerows with elm, and hedgerows in the north of England tend not to follow the rule as closely. The formula also does not work on hedges more than a thousand years old.