
Dear all
I am trying to learn R
I was trying to calculate standard deviation 
here are the commands and outputs in R
> sd(Ht_cm[type=='SD'])[1] 3.283605> sd(Ht_cm[from_treeline=='above'])[1] 16.83289> sd(Ht_cm[type=='SD'][from_treeline=='above'])[1] NA> 

the problem is that, i could not understand why the third command could not give any result. 

please help me out 

thanking you 

Regard
MS
Nepal 		 	   		  
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