A graph which shows the percentage of correct answers given by candidates to each question was devised. Each question is one dot in the Cartesian coordinate system After taking the test the candidates were divided into five groups according to their scholastic ability. In the graph the corret answer percentage of the mediam group is assigned to the abscissa and the difference in the correct answer percentage between the most apt and the median groups are assigned to the ordinate. By the distribution pattern of the dots, the authors tried to analyze, as a whole, how well the test could discriminate the scholastic ability of candidates, as well as how difficult the test was for the candidates to answer.
Graphs for the biology questions on the National Center’s Tests for each year from 1990 to 1995 were made; biology questions were classified according to field and item.
Analytical results obtained from these graphs were as follows: (1) questions about genetics were apparently more discriminating than those about other fields, probably because genetics is the most theoetical among the fields of biology; (2) questions asking about experimental methods or procedures were generally the most difficult for candidates to answer, probably because they had scarcely had training or practice in biology experiments at their high schools; (3) many good questions recommended by high school teachers were less discriminating as far as scholarship and less easy for candidates to answer, while some improper questions which were criticized by the teachers were difficult for candidates to answer but were effective in discriminating among the candidates’ abilities.
