This Data Item provides a method for representing - and, if necessary, relating - the braking performances of aircraft and ground-test machines in wet conditions. The method is essentially statistical and implies that there is a clearly-defined level of probability that can be deduced from test runs of an aircraft (or ground-test machine) in a given set of wetness conditions on a particular runway. If the runway macro-texture depth is known, then the parameter that defines the variation of effective coefficient of braking friction in the given conditions can be readily calculated from a pre-determined Friction Data Base. The theoretical method of representing the coefficient of friction on a wet runway is defined and procedures are given for generating Friction Data Bases. The various methods of using those Friction Data Bases are described and sample results shown. The possibility is considered of interpreting existing Friction Data Bases for aircraft for which no test data are available. Examples of the construction of Friction Data Bases are given in a sequence of three further Data Items. The first example, for a Boeing 727-100QC aircraft, is given in ESDU 99016 and considers a case where an extensive set of test data is available. A second example, for an aircraft where only a limited set of data is available, is given in ESDU 99017. An example of the application of the method to ground-test machine data is given in a ESDU 00018. If the method of this Data Item is to be used to relate the braking performances of a given combination of aircraft and ground-test machine, the procedure is comprised of two distinct phases. The first is the establishment of separate Friction Data Bases for the aircraft and for the ground-test machine. The second is to predict operational braking performance of the aircraft from day-to-day measurements from the ground-test machine.