  We have evaluated forward dispersion relations for scattering amplitudes that
follow from direct fits to several sets of pion-pion scattering experiments,
together with the precise K decay results, and high to energy data. We find
that some of the most commonly used experimental sets, as well as some recent
theoretical analyses based on Roy equations, do not satisfy these constraints
by several standard deviations. Finally, we provide a consistent pion-pion
amplitude by improving a global fit to data with these dispersion relations.
