This page has largely been contributed by John Aldrich of the University of Southampton. Notation for Parameters and Estimates.
Today there are two conventions for representing a parameter and the corresponding estimate.
The other is to use corresponding Greek and Latin characters for parameter and estimate. He used the hat device mostly in conjunction with in general discussions of estimation as in Phil. 1922 - see below. The most familiar example of the Graeco-Latin convention, s, for an estimate of , dates from 1908 but the convention only became established in the 1930s. The growth of the system can be seen through the entries on specific symbols.