Final answer:
A diagnostic assessment should be conducted first to ensure the validity of following assessments by establishing students' current knowledge and skill levels before instruction starts.
Step-by-step explanation:
The assessment that should be conducted first to preserve its validity is the diagnostic assessment. This type of assessment occurs before instruction begins and is designed to determine students' strengths, weaknesses, knowledge, and skills prior to the instruction. Conducting a diagnostic assessment first ensures that the instructor has a baseline understanding of students' prior knowledge, which can inform the teaching approach and avoid any misconceptions or gaps in understanding that might skew the results of subsequent assessments like formative and summative assessments.