added iif and tif functions

app/lib/irt/item_information_function.py

@@ -0,0 +1,14 @@
+from lib.irt.models.three_parameter_logistic import ThreeParameterLogistic
+
+class ItemInformationFunction():
+  def __init__(self, irt_model):
+    self.model_data = irt_model
+
+  def calculate(self, **kwargs):
+    if self.model_data.model == '3PL':
+      p = ThreeParameterLogistic(self.model_data, kwargs).result()
+      q = 1 - p
+      return self.model_data.a_param**2 * ((q / p) * ((p - (self.model_data.c_param**2)) / (1 - (self.model_data.c_param**2))))
+    else:
+      # potentially error out
+      return None

app/lib/irt/test_information_function.py

@@ -0,0 +1,16 @@
+from lib.irt.item_information_function import ItemInformationFunction
+
+class TestInformationFunction():
+  def __init__(self, irt_model):
+    self.irt_model = irt_model
+    self.iif = ItemInformationFunction(irt_model)
+
+  def calculate(self, items, **kwargs):
+    sum = 0
+
+    for item in items:
+      result = self.iif.calculate(b_param=item.b_param, theta=kwargs['theta'])
+      item.iif = result
+      sum += item.iif
+
+    return sum