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help for xtvc 
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Confidence intervals for the variance component of random-intercept linear mode
> ls

        xtvc [, level(#) h0(#) ]

Description

    xtvc is a post-estimation command that presents confidence intervals for
       the variance component of the random effect based on the inversion of a
       score-based test (Bottai, 2003).  It has to be used after xtreg with
       option mle (maximum-likelihood random-effects estimator).

Options

    level(#) specifies the confidence level, in percent, for the confidence
    interval of the variance component; see help level.

    h0(#) performs the score-based test for the null hypothesis H0: sigma_u =
    #.

Remarks

    xtvc only provides interval estimates when the number of units (groups) is
       greater than eight.

Examples

    . webuse nlswork.dta, clear
    . iis idcode
    . xtreg ln_w grade age ttl_exp tenure, mle
    . xtvc
    . xtvc, l(90)
    . xtvc, l(99) h0(0.25)

Authors

    Matteo Bottai, Arnold School of Public Health, University of South
    Carolina, Columbia, USA and Institute of Information Science and
    Technology, National Research Council of Italy, Pisa, Italy

    Nicola Orsini, Institute of Environmental Medicine, Karolinska Institutet,
    Stockholm, Sweden and Institute of Information Science and Technology,
    National Research Council of Italy, Pisa, Italy

Reference

    Bottai, M. (2003) Confidence regions when the Fisher information is zero,
    Biometrika, Volume 90, Issue 1: pp. 73-84.

Also see
 
    Manual:  [U] 23 Estimation and post-estimation commands,
             [U] 29 Overview of Stata estimation commands,
             [XT] xtreg

    Online:  help for estcom, postest, xt, xtreg