A road testing vehicle, SCRIM, has been developed by TRRL to provide a routine method of measuring the resistance to skidding (SFC) of wet roads. Factors affecting SFC measurements made with SCRIM are considered in this Report and estimates are given of their magnitude. These factors, are machine variability including repeatability,, reproducibility, tyre resilience and wear, calibration, recorder drift, tyre pressure and water flow; machine operation including tracking, speed and location; test conditions including water-film thickness, eveness of road surface and turning of SCRIM; and variability associated with the processing of results. There are only relatively small 'errors' associated with the use of a single SCRIM to compare a number of sections during a limited period of time, and steps can be taken to allow more precise comparisons to be made over longer periods.