diff --git a/source/statistics/tests_parametric/ztest.rst b/source/statistics/tests_parametric/ztest.rst
index 07cce64..df18b7e 100644
--- a/source/statistics/tests_parametric/ztest.rst
+++ b/source/statistics/tests_parametric/ztest.rst
@@ -1,2 +1,7 @@
 Z-Test
 -------
+
+One tailed two tailed:
+https://stats.oarc.ucla.edu/other/mult-pkg/faq/general/faq-what-are-the-differences-between-one-tailed-and-two-tailed-tests/
+
+example 2 tailed https://www.mathandstatistics.com/learn-stats/hypothesis-testing/two-tailed-z-test-hypothesis-test-by-hand