Hi, I think you need quotation around I like the following:
status
2010 2011 2012 1 A A A 2 A I I 3 A A A 4 U U U 5 A A A 6 I I I 7 U I I 8 A U A 9 I A U 10 I I I
apply(start,2,function(x) {sum(x=="I",na.rm=TRUE)})
X2010 X2011 X2012
3 4 4
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