## 4.1 Purpose of item bank

The itembank is, as the name implies, a collection of calibrated items (i.e. milestones) that can be used for assessment to measure a construct (i.e. the $$D$$-score). Besides the items, the itembank also holds all information pertaining to those items. The additional item information in the itembank enables the possibility to sort the items based on for example their difficulty or administration mode.

The itembank is used as a repository, from where the items for a test can be extracted. Depending on the purpose of the test, different items may be selected to measure the construct (see @(sec:itemselection)). Any selection of items from the itembank used in a test, leads to a score that can be interpreted in the same way, because all items are based on the same set of item calibrations.