Evaluation samples must be reasonably homogeneous so that subsamples have an insignificant variance of composition. Otherwise, each evaluation sample must be individually certified. The specifications may be more stringent in some cases than for others, and these will depend upon the accuracy requirements for the end use of the sample. The requirement for an evaluation sample is a reasonable match of matrix with that of the usual test samples. Miscible liquids blend perfectly, but stratification of the several constituents can result from the mode of addition. Thus, bulk additions of one liquid into another could result in layers, especially when they differ significantly in density. The spiking of solid samples can take place by either liquid or solid addition. Gases may be blended statically by the mixing of gravimetrically, volumetrically, or manometrically measured quantities. Dynamic blending of two gas streams, in which flow rates are carefully controlled and measured, is an excellent way to produce gas mixtures.