**5. Cronbach's alpha (Index of reliability)**

Cronbach's alpha (KR20) is widely accepted and used estimate of test reliability (the internal consistency) and reported to be superior to the split-half estimate [15, 16]. Although validity and reliability are closely associated, the reliability of an assessment does not depend on its validity [16, 17]. Coefficient alpha is known to be equal to Kr-20 if the item has a single answer, such as in the case of type A MCQs or binary [18–21].

Coefficient alpha reflects the degree to which item response scores correlate with total test scores [15]. It also describes the degree to which items in the exam measure the same concept or construct [22]. Therefore, it is connected to the inter-relatedness and dimensionality of the items within the exam [16, 20]. Cronbach's alpha is affected by exam time, the number and inter-relation of the items (dimensionality) and easy or hard, poorly written or confusing items, Variations in examinee

#### *Item Analysis: Concept and Application DOI: http://dx.doi.org/10.5772/intechopen.100138*

responses, curriculum content not reflected in the test, Testing conditions, and Errors in recording or scoring [22–24]. The value of alpha is decreased in the exam with fewer items and increased if items assessing the same concept (unidimensionality of the exam) [16]. Other factors were reported to impact alpha value, such as item difficulty, number of the examinee, and student performance in the exam time. It was argued that very high alpha values could indicate lengthy exams, parallel items, or a narrow coverage of the content material [22].

The alpha value of the exam can be increased by increasing the number of items with a high p-value (difficulty index). It was reported that items with moderate difficulty could maximize alpha value and while those with zero difficulties or 100 can minimize it [15]. In the same way, deletion of faulty items can increase alpha value. It should be considered that repetition of items in the same exam or using items assessing the same concept can increase alpha value.
