[R] inconsistency in nls output....

[akshay kulkarni]

dear members,
                             I have the following nls output:

 Formula: YLf13 ~ (d + e * ((XL)^(1/3)) + f * log(LM3 + 18.81))

Parameters:
    Estimate Std. Error t value Pr(>|t|)
d  5.892e-09  8.644e-10   6.817 2.06e-11 ***
e -6.585e-09  5.518e-10 -11.934  < 2e-16 ***
f  1.850e-10  2.295e-10   0.806     0.42
---
Signif. codes:  0 �***� 0.001 �**� 0.01 �*� 0.05 �.� 0.1 � � 1

Residual standard error: 9.57e-10 on 677 degrees of freedom

Number of iterations to convergence: 2
Achieved convergence tolerance: 3.973e-08

------
Residual sum of squares: 6.2e-16

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t-based confidence interval:
           2.5%         97.5%
d  4.195378e-09  7.589714e-09
e -7.668142e-09 -5.501342e-09
f -2.655647e-10  6.354852e-10

------
Correlation matrix:
           d             e             f
d  1.0000000 -6.202339e-01 -7.832539e-01
e -0.6202339  1.000000e+00 -2.127301e-05
f -0.7832539 -2.127301e-05  1.000000e+00


if I let XL = 1.1070513 and LM3 = 0.3919 , and consider the coeffs as given above, the right hand side of the above equation is negative.
But YLf13 is always positive! How is this possible? Am I interpreting the result of the nls output properly?  Should I interpret the coeffs differently? I have done hours of thinking over the above problem but couldn't find any results...

I cannot provide the full values of YLf13, XL and LM3 due to IPR issues....please cooperate......however, if the only way to solve the problem is to give these values, I would indeed give them.....

Also forgive me if there is a minor mistake in my calculations... or a typo....

very many thanks for your time and effort....
yours sincerely,
AKSHAY M KULKARNI

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