Hi Bill, Thanks for your response and I'm sorry -- that was a misleading example of what I was trying to show. This one should illustrate the point:
require(AER) data_in = c(0,6,12,18,24,30,36,42,48,54,60,66,72,78) data_in2 = data_in^2 data_in3 = data_in^3 data_out = c(139487.00,133333.00,62500.00,58823.00,56338.00,27549.00,4244.00,600.00,112.00,95.00,48.00,31.00,15.00,14.99) ldata_out = log(data_out) m1 <- lm(ldata_out ~ data_in + data_in2+data_in3) cubic <- tobit(ldata_out ~ data_in + data_in2 + data_in3, left= log(15-0.01),init = coef(m1)) print(cubic) fitted(cubic) n= length(data_in) m2 <- lm(ldata_out[1:(n-1)] ~ data_in[1:(n-1)] + data_in2[1:(n-1)]+data_in3[1:(n-1)]) cubic2 <- tobit(ldata_out ~ data_in + data_in2 + data_in3, left= log(15-0.01),init = coef(m2)) print(cubic2) fitted(cubic2) The models cubic and cubic2 seem to show that the intercept value in the model doesn't change from its starting value: > m1 Call: lm(formula = ldata_out ~ data_in + data_in2 + data_in3) Coefficients: (Intercept) data_in data_in2 data_in3 1.156e+01 1.004e-01 -7.755e-03 6.464e-05 > cubic Call: tobit(formula = ldata_out ~ data_in + data_in2 + data_in3, left = log(15 - 0.01), init = coef(m1)) Coefficients: (Intercept) data_in data_in2 data_in3 11.5559540 0.0289083 -0.0040615 0.0000229 Scale: 3.759 > m2 Call: lm(formula = ldata_out[1:(n - 1)] ~ data_in[1:(n - 1)] + data_in2[1:(n - 1)] + data_in3[1:(n - 1)]) Coefficients: (Intercept) data_in[1:(n - 1)] data_in2[1:(n - 1)] data_in3[1:(n - 1)] 1.150e+01 1.151e-01 -8.381e-03 7.122e-05 > cubic2 Call: tobit(formula = ldata_out ~ data_in + data_in2 + data_in3, left = log(15 - 0.01), init = coef(m2)) Coefficients: (Intercept) data_in data_in2 data_in3 1.150e+01 3.391e-02 -4.187e-03 2.383e-05 Scale: 3.759 Am I missing something here?? -- View this message in context: http://r.789695.n4.nabble.com/different-regression-coeffs-with-different-starting-point-tp3353536p3354276.html Sent from the R help mailing list archive at Nabble.com. ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.