Hi!
I have binary outcome data clustered in individuals and want to use the xtgee command to adjust for correlated residuals within individuals.
I have however problems understanding how using cluster-robust standard errors (in xtgee vce(robust)) and specifying the working correlation matrix in xtgee relate to each other.
According to stata help vce(robust):
I am wondering if there is any use of specifying a within-group correlation structure (the default is exchangeable) if vce(robust) produces "valid standard errors even if the correlations within group are not as hypothesized by the specified correlation structure"?
I am also wondering, what Stata does if an independent correlation structure is specified together with vce (robust). Does Stata just "ignore" my specification and allow for within-group correlation anyway?
Also, is there any reason why someone would want to run a gee with independent working correlation structure, since in my understanding gee is used to adjust for within-group correlation structure?
Thank you!
I have binary outcome data clustered in individuals and want to use the xtgee command to adjust for correlated residuals within individuals.
I have however problems understanding how using cluster-robust standard errors (in xtgee vce(robust)) and specifying the working correlation matrix in xtgee relate to each other.
According to stata help vce(robust):
... specifies that the Huber/White/sandwich estimator of
variance is to be used in place of the default conventional variance
estimator (see Methods and formulas in [XT] xtgee). Use of this
option causes xtgee to produce valid standard errors even if the
correlations within group are not as hypothesized by the specified
correlation structure. Under a noncanonical link, it does, however,
require that the model correctly specifies the mean. The resulting
standard errors are thus labeled "semirobust" instead of "robust" in
this case. Although there is no vce(cluster clustvar) option,
results are as if this option were included and you specified
clustering on the panel variable.
variance is to be used in place of the default conventional variance
estimator (see Methods and formulas in [XT] xtgee). Use of this
option causes xtgee to produce valid standard errors even if the
correlations within group are not as hypothesized by the specified
correlation structure. Under a noncanonical link, it does, however,
require that the model correctly specifies the mean. The resulting
standard errors are thus labeled "semirobust" instead of "robust" in
this case. Although there is no vce(cluster clustvar) option,
results are as if this option were included and you specified
clustering on the panel variable.
I am also wondering, what Stata does if an independent correlation structure is specified together with vce (robust). Does Stata just "ignore" my specification and allow for within-group correlation anyway?
Also, is there any reason why someone would want to run a gee with independent working correlation structure, since in my understanding gee is used to adjust for within-group correlation structure?
Thank you!
Comment