Many psychometricians are also concerned with finding and eliminating test bias from their psychological tests. Test bias is a form of systematic (i.e., non-random) error which leads to examinees from one demographic group having an unwarranted advantage over examinees from another demographic group. According to leading experts, test bias may cause differences in average scores across demographic groups, but differences in group scores are not sufficient evidence that test bias is actually present because the test could be measuring real differences among groups. Psychometricians use sophisticated scientific methods to search for test bias and eliminate it. Research shows that it is usually impossible for people reading a test item to accurately determine whether it is biased or not.