The Test Information Function (TIF) is an extension to the concept of reliability. While reliability is central to Classical Test Theory, TIF is central to Modern Test Theory (Item Response Theory; IRT).
The TIF of Alva’s logic test shows how much information the entire item bank contains. As can be seen in the plot below, most information is located on the high end of the ability scale. This is intentional - the tasks are designed to differentiate well between high-ability individuals.
The TIF is calculated as the sum of Item Information Functions (IIF) for 84 items in Alva’s item bank. The standard formula for calculating Fisher information for the 3 parameter logistic IRT model was used (de Ayala, 2008):
de Ayala, R. J. (2008). The Theory and Practice of Item Response Theory. Guildford Press, New York, US.