fixes for clarity

app/lib/irt/test_response_function.py

@@ -6,6 +6,10 @@ class TestResponseFunction():
     self.irt_model = irt_model
     self.irf = ItemResponseFunction(irt_model)
 
+  # determins the probably of an inidividual
+  # at a certain theta (ability level) would get a sum of questions correct
+  # detailed further on page 166, equation 3 here:
+  # https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5978482/pdf/10.1177_0146621615613308.pdf
   def calculate(self, items, **kwargs):
     sum = 0