Which factor is crucial for ensuring accurate results in slump tests?

To ensure accurate results in slump tests, performing the test on multiple samples is critical because it provides a more comprehensive understanding of the concrete's workability. By testing multiple samples, you can account for any variability in the mix and ensure that the slump measurement reflects the overall consistency of the batch. This approach helps to identify inconsistencies that may arise from different portions of the mix or from variations in the materials used.

While the other factors mentioned may contribute to the integrity of the testing process, they do not directly address the variability within a single batch of concrete as effectively as testing multiple samples does. For instance, while maintaining consistent temperatures is important for the curing process of concrete, it does not directly affect the slump measurement itself. Similarly, using the same person for all tests can minimize operator variability, but it still relies on the quality of individual samples. Timing the test after mixing is necessary to mitigate the effects of setting but does not compensate for variability in the material itself. Thus, testing multiple samples is the most reliable way to ensure the slump results are representative of the entire batch's properties.