The structure of survey design objects changed in version 2.9, to allow
standard errors based on multistage sampling.
as.svydesign converts an
object to the new structure and
.svycheck warns if an object
does not have the new structure.
You can set
options(survey.want.obsolete=TRUE) to suppress the
warnings produced by
options(survey.ultimate.cluster=TRUE) to always compute
variances based on just the first stage of sampling.
Object of class