The purpose of written certification examinations is to identify the candidates who are qualified to practice effectively.  The mechanism for accomplishing this is usually four or five part multiple choice items.  The quality of the multiple choice items included in an examination is the basis for the reliability or the accuracy of the decisions made about candidate performance.  In classical terms, this means the item should have a good p-value (percent correct) and point biserial correlation.  In Rasch terms it means the difficulty, as well as, the infit and outfit should be within acceptable limits.  Of course, the items must reasonably represent the pertinent content areas in the field of practice. Meeting the criteria for good item performance leads to a lower error of measurement, and more accurate outcomes for candidates.  Candidate separation reliability ((Standard Deviation² - Standard Error²)/Standard Deviation²) estimates the accuracy of the measured differences among candidate performance.

On items that are good measures, candidates who do well on the total test have the highest probability of answering the item correctly, while candidates who do poorly have the lowest probability of answering the item correctly.  There are many item writing guides that reiterate item writing principles (see Item Development Guidelines at www.MeasurementResearch.com). When multiple choice items are well written, they distinguish between more and less knowledgeable candidates, reduce the error of measurement, and consequently lead to a higher candidate separation reliability.

One way to reduce measurement error is to include a sufficient number of items on the examination, at least 100.  The conventional wisdom is that more items decrease the error of measurement and increase reliability. However, after reviewing the data from many examinations, we have found that it takes more than long tests to improve reliability.  The consistency of item content within sections and within the test is critical for good reliability.  Another issue is the statistical performance of the item on the test. Whether item performance is measured with classical statistics or with Rasch IRT, items that do not perform well introduce measurement error and subsequently reduce examination reliability.  In fact, we have found that deleting poorly performing items often increases the reliability of the examination, even though the total number of items decreases.  Some examples that confirm the value of deleting poorly performing items are shown in the Table below.