In coal washing this may be expressed as: Efficiency, percent = Actual yield of clean coal X 100 / Theoretical yield at the ash content of the clean coal. The efficiency of separation thus expresses as a percentage, that proportion of the float coal obtained by float-and-sink analysis that will be recovered in practice by a particular washer. The theoretical yield is derived by plotting the cumulative yield of the reconstituted feed coal against the appropriate cumulative ash content and reading off the yield corresponding to the ash content of the clean coal actually obtained.