diff --git a/lib/statsample/reliability.rb b/lib/statsample/reliability.rb
index e5fb50c..ba5deae 100644
--- a/lib/statsample/reliability.rb
+++ b/lib/statsample/reliability.rb
@@ -5,7 +5,6 @@ class << self
       # only uses tuples without missing data
       def cronbach_alpha(ods)
         ds=ods.dup_only_valid
-        return nil if ds.vectors.any? {|k,v| v.variance==0}
         n_items=ds.fields.size
         return nil if n_items<=1
         s2_items=ds.vectors.inject(0) {|ac,v|