  Data from the PHOBOS detector have been used to study two-particle
correlations over a broad range of pseudorapidity. A simple cluster model
parameterization has been applied to inclusive two-particle correlations over a
range of centrality for both Cu+Cu and Au+Au collisions at sqrt(s_NN)=200GeV.
Analysis of the data for Au+Au has recently been extended to more peripheral
collisions showing that the previously-observed rise in cluster size with
decreasing system size eventually reaches a maximum value. Model studies have
been used to quantify the significant effect of limited detector acceptance on
the extracted cluster parameters. In the case of Au+Au, correlations between a
trigger particle with pT>2.5GeV and inclusive associated particles have also
been studied. These reveal the presence of a `ridge' at small relative
azimuthal angle which extends with roughly constant amplitude out to the
largest relative pseudorapidity studied. The large phase-space coverage of the
PHOBOS detector has enabled a quantitative understanding of the so-called
`ZYAM' parameter used in the subtraction of the contribution of elliptic flow
to these triggered correlations.
