validity...

Validity: it is the extent of a test in measuring the skills that were said to be measured. It is necessary that a test is valid in order to interpret and use the results of a test correctly and accurately.
Validity is the evidence of a recorded data. There are several forms of validation.
Content validity: This one validates the content of a test. This test must be made by the material previously taught in the class, if the test contents the right information, it will give us an accurate measure.
Criterion- related validity: This category refers about the assessment of the candidate's ability. There are two kinds: Concurrent validity and predictive validity. Concurrent validity is when the test and the criterion are administered at about the same time, which it will lay on the functions to be test.
Reliability: Consist in coherent results, in other words, the results shown by a test should be similar when it is applied to different students with the same level in the use of a language in different occasions.

Practicality: A test has to be practical,it has to be economical and easy for both, teacher and student, but without stopping being effective.

Backwash effects: It is how students see the test and how they are going to work with it. This perspective of the test will depend on how the teacher manage the instructions and the use of the test with the students; if they feel the test is just another tool for learning, or improve learning, they probably will acquire a positive idea or concept of a test, if not, the results could be the other way around.