An air pollution expert set up a monitoring program to determine the amount of particulate matter (PM) leaving a farmer’s field after crops were harvested and the soil was bare. She set up high-volume air samplers 10 meters from each edge of the square field and collected samples weekly for two months. When she looked at her data, she was surprised by how much the PM levels varied, both from site to site on a given date and over time at each site. Which of the following parameters would have been best to measure to help explain the variation in her results?