Firstmix uses the DOE Method and incorporates the tables and figures contained in the 1988 edition of the DOE Method (see References 2) as data, which is stored for discrete values. Firstmix employs linear interpolation to obtain intermediate values. Since the data values are based on a wide range of test results and cannot be considered exact, the errors caused by linear interpolation will generally be small in relation to the scatter of the test results. Cases where such errors might be significant are noted, for example in the variation of concrete strength with age at testing.