Samples can also be evaluated and quantified for other odor parameters to assist in the odor study. The odor intensity of a sample is the relative strength of the odor above the threshold (supra threshold). The intensity of an odor is referenced on the ASTM Odor Intensity Referencing Scale described in ASTM E544-75,88, Standard Practice for Referencing Supra threshold Odor Intensity. This standard specifies the internationally accepted reference odorant n-butanol. The odor intensity of a sample is expressed in parts per million (ppm) n-butanol. A larger value of n-butanol means a more intense odor. The Odor Intensity Referencing Scale (OIRS) serves as a standard method to quantify the intensity of odors for documentation and comparison purposes.