diff --git a/src/TimeDomain.jl b/src/TimeDomain.jl
index b6ff054..901a995 100644
--- a/src/TimeDomain.jl
+++ b/src/TimeDomain.jl
@@ -10,7 +10,7 @@ This function calculates the differences between the NN intervals
 function nn_diff(n)
     diff=[]
     for i in 1:length(n)-1
-        push!(diff,abs(n[i+1]-n[i]))
+        push!(diff,n[i+1]-n[i])
     end
     return diff
 end #nn_diff
@@ -63,7 +63,7 @@ with an interval smaller than x ms
 function nn(diff,x)
     count=0
     for d in diff
-        if d>x
+        if abs(d)>x
             count+=1
         end
     end
diff --git a/test/runtests.jl b/test/runtests.jl
index ef1ed92..2c600ae 100644
--- a/test/runtests.jl
+++ b/test/runtests.jl
@@ -17,7 +17,7 @@ g=HeartRateVariability.geometric(n)
         @test td.mean≈917.24 atol=0.1
         @test td.sdnn≈137.19 atol=0.1
         @test td.rmssd≈27.85 atol=0.1
-        @test td.sdsd≈20.35 atol=0.1
+        @test td.sdsd≈27.85 atol=0.1
         @test td.nn50≈342 atol=1
         @test td.pnn50≈4.41 atol=0.1
         @test td.nn20≈2831 atol=1