The parameters of type-III sources have been observed to vary as powers of the distance of the source from the Sun. Here, the values of the observed exponents are reviewed and theoretical relationships between them are discussed and extended. It is shown that 11 observed exponents can be derived from a four-element subset. A least-squares fit is carried out by varying these four exponents and it is shown that the results are consistent with observaton to within the observational uncertainties. Best-fit expressions are given for the plasma density and temperature, the solar wind speed, beam velocity and density, frequency drift rate, peak Langmuir fields, brightness temperature, volume emissivity, beam duration and burst decay time.