Two students completed an experiment to determine if pesticide use at local farms was causing fish kills in a local pond. The steps of the experiment are listed below. Select one location to collect water samples in the pond. Collect a 5-mL water sample at 8 am, 12 pm, and 6 pm for 10 days. Measure the amount of pesticide present in each sample using different measurement tools. After analyzing their data, the students conclude that pesticide runoff from farms is causing the fish to die in the pond. Which of these statements best describes why their conclusion is not valid?