One factor that must also be considered is that sometimes there can be random distributions of either vacancies, where an atom is missing from a site, or site substitution, where one element substitutes for another. In some cases, both occur together. If these distributions were ordered, this would mean that the symmetry would be lowered, but if the vacancies and/or substitutions are random, the symmetry is not affected. The effect of vacancies is to reduce the \(f_j\) value linearly, e.g. \(f_j\) is replaced by \(o_j f_j\) where \(o_j\) is a number between 0 and 1. If two or more atoms share a site, one adds extra terms to the summation and the sum of the \(o_j\) values for the atoms that share a site must be between 0 and 1. As will be discussed later, one frequently makes the assumption that the sum is 1 because it requires multiple measurements to demonstrate any other value.