Which of the following measures is critical when handling repeat samples?

When handling repeat samples, it is crucial that they are taken on the same day to ensure consistency and reliability in the results. Sampling conditions can change due to factors such as varying temperatures, changes in water flow, or the introduction of contaminants. By taking repeat samples on the same day, operators can minimize the effects of these variables, thus providing a more accurate assessment of water quality.

This approach allows for a direct comparison between samples, as external influences that could affect the sample's characteristics are controlled. Moreover, taking samples on different days can introduce inconsistencies that might lead to misleading conclusions regarding the water quality or the effectiveness of treatment processes.

Maintaining strict protocols for repeat sampling is vital for regulatory compliance, data integrity, and providing assurance that the water supply meets safety standards.