[gradsusr] Unknown source of errors

[juan pablo sierra perez]

Hello everyone 
I have found differences in the results in this two ways to proceed:
First one: n2=14'define s=ave('var', t='n2'-11, t='n2'-9)' 
Second one:'define s=ave('var', t=3, t=5)' 
How can I use the "average" command with a math expression used to define the times values avoiding the errors?Why is this error produced?
Thanks in advance!
Regrets
Juan  		 	   		  
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