The most difficult task on the prose literacy scale (377) requires readers to look at an announcement from a personnel department and to "list two ways in which CIEM (an employee support initiative within a company) helps people who lose their jobs because of departmental reorganization." Type of match was scored 7 because the question contained multiple phrases that the reader needed to keep in mind when reading the text. In addition, readers had to provide multiple responses and make low text-based inferences. Type of information was scored 3 because readers were looking for a purpose or function, and plausibility of distractor was scored a 4. This task is made somewhat more difficult because the announcement is organized around information that is different from what is being requested in the question. Thus, while the correct information is listed under a single heading, this information is embedded under a list of headings describing CIEM's activities for employees looking for other work. Thus, the list of headings in this text serves as an excellent set of distractors for the reader who does not search for or locate the phrase in the question containing the conditional information—those who lose their jobs because of a departmental reorganization.

Evaluating the contribution of the variables to task difficulty

The Item Response Theory (IRT) scaling procedures that were used in the IALS constitute a statistical solution to the challenge of establishing one or more scales for a set of tasks with an ordering of difficulty that is essentially the same for everyone. Each scale can be characterized in terms of how tasks are ordered along it. The scale point assigned to each task is the point at which individuals with that proficiency score have a given probability of responding correctly. In IALS, a response probability of 80% (RP80) was used. This means that individuals estimated to have a particular scale score are expected to perform tasks at that point on the scale correctly with an 80% probability. It also means they will have a greater than 80% chance of performing tasks that are lower on the scale. It does not mean, however, that individuals with given proficiencies can never succeed at tasks with higher difficulty values; they may do so some of the time. It does suggest that their probability of success is "relatively" low—that is, the more difficult the task relative to their proficiency, the lower the likelihood of a correct response.

An analogy might help clarify this point. The relationship between task difficulty and individual proficiency is much like the high jump event in track and field, in which an athlete tries to jump over a bar that is placed at increasing heights. Each high jumper has a height at which he or she is proficient—that is, the jumper can clear the bar at that height with a high probability of success, and can clear the bar at lower heights almost every time. When the bar is higher than the athlete's level of proficiency, however, it is expected that the athlete will be unable to clear the bar consistently.

Once the literacy tasks are placed along each of the scales using the criterion of 80% (RP80), it is possible to see to what extent the variables associated with task characteristics explain the placement of tasks along the scales. A multiple regression was run using RP80 as the dependent variable.³ The independent variables were the three process variables (TOM, TOI, and POD) used to characterize the prose tasks, plus a traditional measure of readability⁴ (READ). The results are shown here in Table 1.

Table 1 shows the zero order correlation of each predictor variable with RP80 along with the results of the regression analysis. These data reveal that type of match had the largest zero order correlation with RP80 (.89) and received the largest standardized regression weight, followed by plausibility of distractor and type of information. Together these variables, along with readability, accounted for 89% of the variance in predicting RP80 values.