A concerned mother wants to ensure that her children are drinking pure water. Specifically, she wants to compare the water quality that comes from her kitchen sink to that which runs through her stand-alone water-filtration system. She purchases a device that measures the contamination of the water on a scale of 0–100, where 0 indicates no contaminants and 100 means the water is fully contaminated and unsuitable for human consumption.
She selects 30 samples of water from her kitchen sink. She randomly selects 15 of the samples to run through the water-filtration system. Afterward, she measures the contamination of all 30 samples of water, cleaning the device between each measurement.
She computes the average contamination score for the 15 samples of water that came directly from her sink, as well as the average contamination score for the 15.

What is the purpose of the random assignment?
to ensure that there are enough samples of water in each group
to ensure that the two groups of water are of equal sizes
to ensure that the two groups of water samples are roughly equivalent at the beginning of the experiment
to reduce variability in the water contamination levels