--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/tests/test_error.py	Sun Dec 17 13:22:44 2017 -0800
@@ -0,0 +1,29 @@
+#!/usr/bin/env python
+
+"""
+test error computation functionality
+"""
+
+import numpy as np
+import os
+import unittest
+from tvii import error
+
+class ErrorTest(unittest.TestCase):
+
+    def test_error_computation(self):
+        """test error computation"""
+
+        def square(x):
+            return x*x
+
+        x = [[1], [2], [3]]
+        y = np.array([0, 3, 8])
+
+        yhat, _err = error.error(square, x, y)
+
+        assert list(yhat) == [1, 4, 9]
+        assert list(_err) == [1, 1, 1]
+
+if __name__ == '__main__':
+    unittest.main()

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/tvii/error.py	Sun Dec 17 13:22:44 2017 -0800
@@ -0,0 +1,12 @@
+import numpy as np
+
+def error(function, x, y):
+    """
+    computes true value and error and returns
+    them both as a 2-tuple
+    """
+
+    yhat = np.array([function(*val) for val in x])
+    error = yhat - np.array(y)
+
+    return (yhat, error)