Analysis of the discrimination index of final biology examinations in Malta

Abstract

Item analysis is a range of statistics that helps to determine the effectiveness of each item in an examination. It plays an important role in contributing to the fairness of the examination as well as helps to identify content areas that may be problematic for students. The validity and reliability of an examination finally depend on the characteristics of the items. Item analysis permits a high reliability and validity to be built into a test in advance. Discrimination index (D) is part of item analysis that measures the difference in item difficulty between groups of students with high and low marks. The index varies between -1 and 1 where the item ideally should be between +0.3 and +1.0. A highly discriminating item is indicative of students who gained high tests scores got the item correct whereas those who had low test scores got the item incorrect. The objective of our study was to calculate the discrimination indices of Advanced level Biology final examinations at a public post-secondary institution in Malta. The final scores obtained by first-year students over a five-year period (n = 1315), 2014-2018, in Papers 1 (short-type items) and 2 (comprehension, structured and unstructured essays) were used to calculate the discrimination index for each item. Results are encouraging since negative discrimination, indicative of a defective item, were not observed in any of the items. Paper 1 is better at discriminating between high and low achievers since over the study period, 93% of the items had acceptable (D between 0.2-0.29) or good discrimination (D between 0.3-0.39) while less, 54%, in Paper 2. Also, fewer (5%) of the items in Paper 1 had poor discrimination (D < 0-0.19) but 46% in Paper 2. Results show that comprehension items are better than the essay type to discriminate between high and low achievers. This finding may be used to start a discussion at the institution to consider the validity of the essay-type of items in final Biology examinations.