The rate of a reaction is always positive. A negative sign is present to indicate that the reactant concentration is decreasing. The IUPAC recommends that the unit of time should always be the second. The rate of reaction differs from the rate of increase of concentration of a product P by a constant factor (the reciprocal of its stoichiometric number) and for a reactant A by minus the reciprocal of the stoichiometric number. The stoichiometric numbers are included so that the defined rate is independent of which reactant or product species is chosen for measurement. For example, if "a = 1" and "b = 3" then B is consumed three times more rapidly than A, but v "= -d["A"]/dt = -(1/3)d["B"]/dt" is uniquely defined. An additional advantage of this definition is that for an elementary and irreversible reaction, "v" is equal to the product of probability of overcoming the transition state activation energy and the number of times per second the transition state is approached by reactant molecules. When so defined, for an elementary and irreversible reaction, "v" is the rate of successful chemical reaction events leading to the product.