Another common modification is the stretched exponential form k = A exp ⁡ ⁡ [ − − ( E a R T ) β β ] , where β is a dimensionless number of order 1. This is typically regarded as a purely empirical correction or fudge factor to make the model fit the data, but can have theoretical meaning, for example showing the presence of a range of activation energies or in special cases like the Mott variable range hopping.