Final answer:
The two main types of validity that an assessment instrument may possess are content validity and criterion-related validity.
Step-by-step explanation:
The two main types of validity that an assessment instrument may possess are content validity and criterion-related validity. Content validity refers to the extent to which the content of the assessment instrument represents the construct being measured. For example, if the instrument is measuring knowledge of history, the questions should be relevant to history. Criterion-related validity, on the other hand, refers to the extent to which the scores on the assessment instrument can predict or correlate with a criterion of interest. For example, if a test is designed to predict students' performance in college, the scores on the test should be correlated with their actual college grades.