diff --git a/lib/mutant/reporter/cli/printer.rb b/lib/mutant/reporter/cli/printer.rb
index 3e972a45..4d5f8235 100644
--- a/lib/mutant/reporter/cli/printer.rb
+++ b/lib/mutant/reporter/cli/printer.rb
@@ -91,6 +91,8 @@ module Mutant
         # Test if output can be colored
         #
         # @return [Boolean]
+        #
+        # @api private
         alias_method :color?, :tty?
       end # Printer
     end # CLI
diff --git a/lib/mutant/result.rb b/lib/mutant/result.rb
index b207129e..509fcb9c 100644
--- a/lib/mutant/result.rb
+++ b/lib/mutant/result.rb
@@ -203,6 +203,11 @@ module Mutant
         test_result.runtime
       end
 
+      # The time spent on killing
+      #
+      # @return [Float]
+      #
+      # @api private
       alias_method :killtime, :runtime
 
       # Test if mutation was handled successfully