Task and characteristic curve: seeing difficulty and discrimination

In brief

The characteristic curve of a test task shows how likely a right answer is at every ability. Two numbers shape it: the difficulty b (the ability at which the probability is 50%) and the discrimination a (how steeply the curve rises there). With the sliders two tasks and one person can be compared.

Rasch and 2PL

In the Rasch model all tasks have the same steepness, the curves just lie next to each other. In the 2PL model each task may have its own steepness, curves can cross. Then one task is no longer "harder" than the other, it depends on the ability. Statable calculates both models below the scale card "How reliably does my scale measure?".