In pharmacology, the international unit (IU) is a unit of measurement for the effect, not mass, of a substance; the variance is based on the biological activity or effect, for the purpose of easier comparison across similar forms of substances. International units are used to quantify vitamins, hormones, some medications, vaccines, blood products, and similar biologically active substances. Many biological agents exist in different forms or preparations (e.g. vitamin A in the form of retinol or beta-carotene). The goal of the IU is to be able to compare these, so that different forms or preparations with the same biological effect will contain the same number of IUs. To do so, the WHO Expert Committee on Biological Standardization provides a reference preparation of the agent, arbitrarily sets the number of IUs contained in that preparation, and specifies a biological procedure to compare other preparations of the same agent to the reference preparation. Since the number of IUs contained in a new substance is arbitrarily set, there is no equivalence between IU measurements of different biological agents. For instance, one IU of vitamin E cannot be equated with one IU of vitamin A in any way, including mass or efficacy. The term international unit is also used to describe any of various physical or monetary units agreed upon internationally, in particular one forming part of the International System of Units (SI). International units as used in pharmacology are not part of the SI. The IU is distinct from the enzyme unit, also known as the international unit of enzyme activity and abbreviated as U. Equality and equivalency of IU for different substances To define the IU for a substance, an international collaborative study is organized by the WHO Expert Committee on Biological Standardization using various assay systems in several laboratories. The main goal of the study is to reach a consensus regarding methods of analysis and the approach to standardization so that results can be compared. The study is performed using highly purified preparations of the substance, typically in lyophilized form, called "international reference preparations" or IRPs. Each preparation is divided into precisely weighed samples, with each sample stored in its own ampoule labeled with a code corresponding to the source IRP. Assays are performed using these samples and are calibrated against the previously available IU standard. These results can be quite variable; the final IU value for samples of a given