A strong signal for double parton scattering (DP) is observed in a
16 pb^-1 sample of pbar p -> gamma + 3 jets + Chi data from the CDF experiment
at the Fermilab Tevatron.  The process-independent DP parameter, sigma_eff, is
obtained without reference to theoretical calculations by comparing observed
DP events to events with hard scatterings at separate pbar p collisions.  The
result, sigma_eff = (14.5 +\- 1.7^1.7_ -2.3) mb, represents a significant
improvement over previous measurements.  For the first time, the Feynman x
dependence of the sigma_eff parameter is investigated, and no dependence is
seen.