In different standards involving reverberation chambers, the field strength is defined either using the magnitude of the vector (total) electric field, or the magnitude of a Cartesian (rectilinear) field component. We compare these methods for determining the maximum field strength. The ratio of the maximum of the total field to the maximum of a Cartesian component is a random variable whose probability distribution and statistics are derived and investigated. The mean value of this ratio is compared with the ratio of the respective mean values. Its standard deviation enables the estimation of confidence levels when comparing or converting between single measurements of the maximum total and maximum rectangular field. Theoretical results are illustrated and compared with measured results obtained in four different reverberation chambers.