outlier takes a "robust2sls" object and the index of a specific
observation and returns its history of classification across the different
iterations contained in the "robust2sls" object.
outlier(robust2sls_object, obs)outlier returns a vector that contains the 'type' value for
the given observations across the different iterations. There are three
possible values: 0 if the observations is judged to be an outlier, 1 if not,
and -1 if any of its x, y, or z values required for estimation is missing.
An object of class "robust2sls".
An index (row number) of an observation