Since each multiple-choice problem had five choices, random guessing on five such problems would produce, on average, one correct answer and four incorrect answers, for a total adjusted score of 0. The effect of this scoring method is shown in the graph below. For example, a person who answered 30 total problems correctly also, on average, answered 4 multiple-choice problems incorrectly. We can use the guessing adjustment curve above to bias the Rasch ability curve as shown in the following graph. The following graph shows the item easiness as determined by the Rasch analysis.
Higher values mean easier problems.