Statistical factors for chemical reactions can be defined in two ways. If all identical atoms in the reactants are labelled, the definition can include (i) the total number of different sets of products that can be formed, or (ii) only those sets of products that are formed by chemically plausible processes. It is proved rigorously that with either definition the ratio of the statistical factors for a reaction in forward and reverse directions is equal to the ratio of the products of symmetry numbers for reactants and products.