The dyads in the network are stratified according to an attribute combination. This typically results in stratifying proposals by mixing type on a vertex attribute.
# strat(attr=NULL, pmat=NULL, empirical=FALSE)
The user may pass a vertex attribute
attr as an argument
(the default for
attr gives every vertex the same attribute
value), and may also pass a matrix of weights
pmat (the default
pmat gives equal weight to each mixing type). See
Specifying Vertex Attributes and Levels for details on specifying vertex attributes. The
pmat , if specified, must have the same dimensions as a
mixing matrix for the network and attribute under consideration,
and the correspondence between rows and columns of
attr is the same as for a mixing matrix.
The interpretation is that
pmat[i,j]/sum(pmat) is the probability of
proposing a toggle for mixing type
(i,j) . (For undirected, unipartite
pmat is first symmetrized, and then entries below the diagonal
are set to zero. Only entries on or above the diagonal of the symmetrized
pmat are considered when making proposals. This accounts for the
convention that mixing is undirected in an undirected, unipartite network:
a tail of type
i and a head of type
j has the same mixing type
as a tail of type
j and a head of type
As an alternative way of specifying
pmat , the user may pass
empirical=TRUE to use the mixing matrix of the network beginning
the MCMC chain as
pmat . In order for this to work,
that network should have a reasonable (in particular, nonempty) edge set.
While some mixing types may be assigned zero proposal probability
(either with a direct specification of
pmat or with
this will not be recognized as a constraint by all components of
and should be used with caution.
ergmHint for index of constraints and hints currently visible to the package.
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