[Rsiena-help] What does NA for standard errors mean?

[tanmay sinha]

Hi all, I am a new user to RSiena. After running the model that includes
certain structural configurations, I find that some of my standard errors
are NA.
I include a portion of the output, with the 3 columns representing
estimates, standard errors and t-ratios.

   8. eval GWESP I -> K -> J (69)               2.1135  (     NA   )
-0.7091
   9. eval GWESP I <- K <- J (69)               0.1239  (     NA   )
-0.1293
  10. eval GWESP I <- K -> J (69)              -0.4884  (     NA   )
-0.0660
  11. eval GWESP I -> K <- J (69)              -0.4797  (     NA   )
-0.1918


What do the NA mean?? How do I interpret this?

I would greatly appreciate your help.

Regards
Tanmay Sinha
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