Definition : A diagnostic test is a test given at the beginning of a course which aims to discover exactly what the learners know or don't know already and where their strengths and weaknesses lie. This information is then used to decide the course content. Diagnostic tests are therefore formative in function.
Example 1 : A teacher in a private language school has a new group of learners. In the school placement test, they all tested out at CEFR B1 level. However, they have all studied in different contexts previously, and the teacher does not therefore know exactly what they have covered. She therefore designs a diagnostic test which covers :
a) the language items and subskills usually encountered at A2 level. Their performance on this part of the test will tell her which items they have fully assimilated and which will need to be focused on before moving on.
b) a range of items and subskills usually encountered in the early stages of a B1 course and covered in the textbook that she intends to use. Their performance on these items will tell her whether they in fact do know some of the items that she would expect to have to teach from scratch, but which in fact will only need to be recycled and consolidated.
Example 2 : A CEFR C1 level learner is attending a one-to-one tailor made Business English course to improve his ability to negotiate in English. A diagnostic test is designed which focuses on language items and subskills commonly needed in negotiations, and including a negotiations simulation which is filmed and analysed by the teacher. From this, the teacher can identify the learner's weak areas, and design a course to meet his needs.