In the current mathematical notation, the differential formula 4 is an inexact differential. These procedures give inexact results&colon; they represent a compromise between accuracy and computation time required. Thus, infinitesimal increments of heat and work are inexact differentials. Information theory recognises that all data are inexact and statistical in nature. Ethics, unlike some other types of philosophy, is inexact and uncertain. It follows that a calculation of the type is even more inexact. Matches can be complete or partial, and exact or inexact. It was Snellius who noted that Ptolemy's equation for refraction was inexact. The lowercase Greek letter delta, , is the symbol for inexact differentials. Categorizing cheeses by moisture content or firmness is a common but inexact practice. American scientists found this method inexact. The irregular and inexact rhymes and varied lengths of the lines play some part. This example captures the essential idea behind the inexact differential in one dimension. Trainers recognise that lipreading is an inexact art. Typically this is determined as the final result being inexact.