In addition, the peakedness of the IIF plots is also useful in that items with steep, narrow, peaked IIF curves denote that the item is highly informative over a specific range of ability. In contrast, shallow, less-peaked IIF curves denote items where a lesser amount of information is spread out over a wider range of ability levels.
While the Item Information Function (IIF) represents the range of ability levels that each individual item is most informative over, the Test Information Function (TIF) represents the range of ability levels that the test as a whole is most informative over, and over which it functions most effectively. Just as the Item Information Function is related to how precise a given individual item is at different ability levels, the Test Information Function is related to how precise the test is across different ability levels.
This overall accuracy and precision is indexed through the inverse of the Standard Error of θ, which simply quantifies the expected error for any estimate along the range of ability levels (θ). In practical terms, when the TIF curve is concentrated over a belowaverage ability level (θ < 0), the test is most effective and provides estimates with lowest standard error for individuals with lower ability levels. When the TIF is concentrated (peaked) over higher ability levels (θ > 0), this indicates the test as a whole is most effective at evaluating above-average ability levels.
We make sure that the TIF plot is always reviewed to verify that our tests are effective in relation to the desired range of ability levels.