  To constrain cosmological parameters, one often makes a joint analysis with
different combinations of observational data sets. In this paper we take the
figure of merit (FoM) for Dark Energy Task Force fiducial model (CPL model) to
estimate goodness of different combinations of data sets, which include 11
widely-used observational data sets (Type Ia Supernovae, Observational Hubble
Parameter, Baryon Acoustic Oscillation, Cosmic Microwave Background, X-ray
Cluster Baryon Mass Fraction, and Gamma-Ray Bursts). We analyze different
combinations and make a comparison for two types of combination based on two
types of basic combinations, which are often adopted in the literatures. We
find two sets of combinations, which have strong ability to constrain the dark
energy parameters, one has the largest FoM, the other contains less
observational data with a relative large FoM and a simple fitting procedure.
