A way of measuring a quantity based on the logarithm function, f(x) = log(x). The logarithm function converts a ratio to a difference: log (a/b) = log a – log b. This is very useful for working with growth rates. For instance, if national income doubles from 50 to 100 in a poor country and from 1,000 to 2,000 in a rich country, the absolute difference in the first case is 50 and in the second 1,000, but log(100) – log(50) = 0.693, and log(2,000) – log(1,000) = 0.693. The ratio in each case is 2 and log(2) = 0.693.