Expeience has shown that the vast majority of measurement variations are due to procedural differences. Something has changed or a procedure is being performed in a different manner. In addition, factors such as the relative humidity level, conditioning time, age of samples, migration of antistatic agents, additive type, test voltage, cutoff level, equipment used, data analysis and technical expertise can all contribute to differences. Material formulation may contribute to some erroneous readings and on rare occasions, equipment problems are involved. Many ESD materials change over time and with humidity. This dynamic characteristic may make it difficult to obtain similar results when taken at different times. The majority of ETS test equipment is supplied with a system check module or test fixture which is used to verify the functioning and accuracy of ETS equipment before testing begins and at different locations.