A true material comparison is possible only when property values are determined by identical test methods under identical conditons. Generally speaking, physical and mechanical properties of plastics and electrical insulating material are affected by temperature and humidity. Plastic materials tested above room temperature will yield relatively higher impact strength and lower tensile strength and modulus. Obviously, in order to make reliable comparisons of different materials and test results obtained by different laboratories, it is necessary to establish standard conditions of temperature and humidity for testing.