On Saturday,  6 Jan 2018 at 09:37, Uwe Brauer wrote:
> The column of reference is E1 per default, which is the second one. The

Ah, okay, I understand now.  Thanks.

I see that you have been given a solution elsewhere using vmean but
you could also consider adding "N" to the expression (i.e. after
;E).  This would make the empty entries be treated as having a 0 value
so your check for nan would no longer work but you could always check
for 0...  The following works:

| Name   | E1 | E2 | E3 | E4 | Res |
|--------+----+----+----+----+-----|
| Entry1 |    |    |    |    |  NP |
| Entry2 | 10 | 20 | 30 | 40 |  10 |
| Entry3 | 10 |    | 20 | 30 |   6 |
#+TBLFM: $6=if("$2" == "(0)" , string("NP"),($2+$3+$4+$5)/10);EN

HTH,
eric

-- 
Eric S Fraga via Emacs 27.0.50, Org release_9.1.6