Hi all,
I fit a Tobit model to Fluid milk consumption (dependent variable) data
using survreg. 1) The square root of the dependent variable was used to
correct for
heteroscedasticity since it provides the best fit. 2) The output before
transformation of numeric variables in scores (by "Make.Z") was:
survreg(formula=Surv(QUALEITERPER, QUALEITEPER>0,type= "left") ~ SALARPER +
IDADE +
IDADE2 + NUMORAD + CIGAFAI + LOCAL, data = leite, dist = "gaussian")
Value Std.Error z p
(intercept) 14.20 2.50 5.58 1.34e-08
SALARPER 2.98 0.87 3.41 6.45e-04
IDADE -0.50 0.06 -7.91 2.48e-15
IDADE2 0.006 0.00 7.37 1.76e-13
NUMORAD -0.61 0.25 -2.39 1.69e-02
CIGAFAI-10 2.20 2.04 1.11 2.68e-01
CIGAFAI-NF 4.66 1.18 3.92 8.80e-05
LOCALh 4.13 1.82 2.27 2.35e-02
LOCALi 1.78 1.62 1.09 2.74e-01
LOCALm 3.66 1.93 1.89 5.82e-02
LOCALt 4.66 1.85 2.51 1.20e-02
Log(scale) 2.26 0.03 73.35 0.00e+00 Scale=9.65 Gaussian distribution
Loglik(model)=-2270.8 Loglik (intercept only)=-2335
Chisq=128.5 on 10 degrees of
freedom, p=0
Number of Newton-Raphson Iterations: 4
N=704 (19 observations deleted due to missingness)
2 - The output after transformation of numeric variables in scores by
"Make.Z(X)" was: survreg(formula=Surv(QUALEITERPERz,
QUALEITEPERz>-1.4977406,type= "left") ~ SALARPERz +
IDADEz + IDADE2z + NUMORADz + CIGAFAI + LOCAL, data = leite, dist =
"gaussian")
Value Std.Error z p
(intercept) -0.80 0.20 -3.93 8.38e-05
SALARPERz 0.15 0.04 3.41 6.45e-04
IDADEz -1.13 0.14 -7.91 2.48e-15
IDADE2z 1.03 0.14 7.37 1.76e-13
NUMORADz -0.11 0.04 -2.39 1.69e-02
CIGAFAI-10 0.24 0.22 1.11 2.68e-01
CIGAFAI-NF 0.51 0.13 3.92 8.80e-05
LOCALh 0.45 0.20 2.27 2.35e-02
LOCALi 0.19 0.17 1.09 2.74e-01
LOCALm 0.40 0.21 1.89 5.82e-02
LOCALt 0.51 0.20 2.51 1.20e-02
Log(scale) 0.06 0.03 1.95 5.16e-02 Scale=1.06 Gaussian distribution
Loglik(model)=-993.3 Loglik (intercept only)=-1057.6
Chisq=128.5 on 10 degrees of freedom, p=0
Number of Newton-Raphson Iterations: 4
N=704 (19 observations deleted due to
missingness)
I am confused about the output, because: 1) I do not know how to interpret
the "scale". 2) The scale/log(scale) interferes in the interpretation of
coefficients or both
are independents? So, how I interpret the coefficients? Give me one
example. 3) How I interpret the coefficients (after score z
transformation)? Give me one
example. 4) How I could interpret pseudo R2 if it modifies after score z
transformation?
Should I take it in consideration? Is there some alternative? Thanks,
Marcio Roberto Silva
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