diff --git a/rowers/datautils.py b/rowers/datautils.py
index 8f35ea1f..4c96f0ca 100644
--- a/rowers/datautils.py
+++ b/rowers/datautils.py
@@ -297,7 +297,7 @@ def getmaxwattinterval(tt,ww,i):
         try:
             t_0 = tt.ix[indexmax]
             t_1 = tt.ix[indexmax-i]
-            deltas = tt.ix[indexmax-1:indexmax].diff().dropna()
+            deltas = tt.ix[indexmax-i:indexmax].diff().dropna()
             testres = 1.0e-3*deltas.max() < 30.
             if testres:
                 deltat = 1.0e-3*(t_0-t_1)