  We input solar wind parameters responsible for the main phases of 15 great
geomagnetic storms (GGSs: $\Delta$SYM-H$\le-$200 nT) into the empirical
formulae created by \cite{Burton1975}(hereafter Burton equation), and by
\cite{OBrien2000}(hereafter OM equation) to evaluate whether \textbf{two
equations} can correctly estimate the intensities of GGSs. The results show
that the intensities of most GGSs estimated by OM equation are much smaller
than the observed intensities. The RMS error between the intensities estimated
by OM equation and the observed intensities is \textbf{203} nT, implying that
the estimated storm intensity deviates significantly from the observed one. The
RMS error between the intensities estimated by Burton equation and the observed
intensities is 130.8 nT. The relative error caused by Burton equation for the
storms with intensities $\Delta$SYM-H$<$-400 nT is larger than 27\%, implying
that the absolute error will be large for the storms with $\Delta$SYM-H$<$-400
nT. The results indicate that the two equations cannot work effectively in the
estimation of GGSs. On the contrary, the intensity of a GGS estimated by the
empirical formula created by \cite{WangCB2003} can always be very close to the
observed one if we select the right weight for solar wind dynamic pressure,
proving that solar wind dynamic pressure is an important factor for GGS
intensity, but it is overlooked in the ring current injection terms of Burton
equation or OM equation. This is the reason why the two equations cannot work
effectively in the estimation of GGSs.
