A researcher is comparing three new formulas for grass killer. To conduct the experiment, the researcher purchases 30 pieces of sod from a local vendor and selects the treatment groups by placing slips of paper numbered 1–30 in a hat. To make three groups for the three formulas, the researcher randomly selects 10 numbers, making sure to mix the slips between each selection, to get the formula 1 group. Then the researcher selects 10 more numbers to get the formula 2 group. The remaining 10 numbers make up the formula 3 group.