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From |
Hitesh Chandwani <hchandwani.stata@gmail.com> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
st: Trying to compare means and using xi and xi3 for survey data |

Date |
Mon, 4 Jul 2011 23:08:01 -0400 |

Hello Statalisters, I am using cost survey data and have 2 questions: 1) Comparison of means Using the svy: mean procedure, I can get means of cost for all categories of a particular variable. But since this variable is not dichotomous, using -test- or -lincom- as a postestimation command to compare the means, doesn't yield any results. What I thought of was dummy coding the categories and then running a regression. Instead of manually creating dummy variables, I decided to use -xi-; which brings me to my next question, 2) -xi- and -xi3- will both omit one category as a reference category..which is fine. But, in my output, after omitting the first category, another category is indicated as (dropped). Moreover, there is still no value for the F-statistic. Firstly, is my approach correct? And secondly, why are 2 categories being dropped? (One explanation that I could come up with for the 2 dropped categories is that the pweight for the observations in the omitted category " _Iinsured_p_0" is set to zero and hence Stata needs to use another category as reference) The following is my syntax as well as output: xi: svy: regress totchg_num i.insured_pub_pvt_un i.insured_pub~n _Iinsured_p_0-4 (naturally coded; _Iinsured_p_0 omitted) (running regress on estimation sample) Survey: Linear regression Number of strata = 75 Number of obs = 103817 Number of PSUs = 966 Population size = 469088.57 Design df = 891 F( 3, 889) = . Prob > F = . R-squared = 0.0106 ------------------------------------------------------------------------------ | Linearized totchg_num | Coef. Std. Err. t P>|t| [95% Conf. Interval] -------------+---------------------------------------------------------------- _Iinsured_~1 | 6504.334 915.0348 7.11 0.000 4708.46 8300.209 _Iinsured_~2 | (dropped) _Iinsured_~3 | -3015.988 705.0121 -4.28 0.000 -4399.666 -1632.31 _Iinsured_~4 | 1070.352 1961.327 0.55 0.585 -2779.007 4919.711 _cons | 13894.47 837.4082 16.59 0.000 12250.95 15538 ------------------------------------------------------------------------------ . test _Iinsured_p_1 _Iinsured_p_2 _Iinsured_p_3 _Iinsured_p_4 Adjusted Wald test ( 1) _Iinsured_p_1 = 0 ( 2) _Iinsured_p_2 = 0 ( 3) _Iinsured_p_3 = 0 ( 4) _Iinsured_p_4 = 0 Constraint 2 dropped F( 3, 889) = 23.78 Prob > F = 0.0000 Any help in understanding this issue will be greatly appreciated. Regards, -- Hitesh S. Chandwani University of Texas at Austin * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

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