[Limdep Nlogit List] Multiple Wald-tests for linear restrictions

[Rady Talaat]

Hello,
I am investigating the best specification for the attributes. The non-linear dummy codes were assigned to attribute by attribute but the linear effects format was retained for the remaining attributes to test whether an attribute should be specified as being either linear or non-linear. Multiple Wald-tests for linear restrictions were performed to examine the specification of the experiment attributes. However I got the following error message for some tests:
Error 310: Computing restricted least squares. R*VAR(b)*Rt singular.
What does it mean? What should I do to avoid it and perform the test?
Cheers,
Rady
 
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