The time evolution of a series project activity measures can be characterised by the the Hurst exponent. A random walk produces a time-series with a Hurst exponent close to 0.5 -for a process with long-term memory the value is higher, and lower for an antipersistent process. Harold Hurst was a hydrologist working in Egypt from 1906. He was interested in the yearly variation in the levels of the Nile and he noticed that the system, rather than being simply periodic or random, showed clustering of flood years and drought years. He devised a way of numerically characterising a time-series of observations as a single number, the Hurst exponent which takes values between 0 and 1. The Hurst exponent must be estimated from a time-series and the estimate is generally considered unreliable for series with fewer than a hundred points . The Hurst exponents for time ordered series of PCA log ED50 activity values were calculated by the rescaled range method 24 . The distribution of Hurst exponents for 100 Syngenta herbicide projects is shown below (figure ), the mean value is 0.72 and the standard deviation 0.11.