Final answer:
Screening instrument validity is influenced by the measuring device's limitations, the measurer's skill, irregularities in the object, and situation-specific factors. Validity ensures accurate measurement, while reliability ensures consistent results.
Step-by-step explanation:
Factors affecting screening instrument validity include the limitations of the measuring device, the skill of the person making the measurement, irregularities in the object being measured, and any other factors that affect the outcome, which can be highly dependent on the situation. Validity is crucial for ensuring that a screening tool accurately measures what it is supposed to measure, distinguishing it from reliability, which is the tool's ability to produce consistent results over time. Essential aspects to consider are the relevance of the results to the real-world properties, the effectiveness of the screening criteria, the speed and robustness of the procedure, and the accuracy and reproducibility of the results.