Fix mirr function and its test. Thanks go to Skipper and Josef.

1 files changed, 4 insertions, 8 deletions

diff --git a/numpy/lib/financial.py b/numpy/lib/financial.py
index 5d1e65f5c..503d43647 100644
--- a/numpy/lib/financial.py
+++ b/numpy/lib/financial.py
@@ -626,16 +626,12 @@ def mirr(values, finance_rate, reinvest_rate):
     """
 
     values = np.asarray(values, dtype=np.double)
-    initial = values[0]
-    values = values[1:]
     n = values.size
     pos = values > 0
     neg = values < 0
     if not (pos.any() and neg.any()):
         return np.nan
-    numer = np.abs(npv(reinvest_rate, values*pos))
-    denom = np.abs(npv(finance_rate, values*neg))
-    if initial > 0:
-        return ((initial + numer) / denom)**(1.0/n)*(1 + reinvest_rate) - 1
-    else:
-        return ((numer / (-initial + denom)))**(1.0/n)*(1 + reinvest_rate) - 1
+    numer = np.abs(npv(reinvest_rate, values*pos))*(1 + reinvest_rate)
+    denom = np.abs(npv(finance_rate, values*neg))*(1 + finance_rate)
+    return (numer/denom)**(1.0/(n - 1))*(1 + reinvest_rate) - 1
+