Let's say you want to know if the type of soil affects how long it takes a seed to germinate, and you decide to set up a controlled experiment to answer the question. You might take five identical pots, fill each with a different type of soil, plant identical bean seeds in each pot, place the pots in a sunny window, water them equally, and measure how long it takes for the seeds in each pot to sprout. The types of soil are regular topsoil, topsoil with sand, topsoil with rocks, topsoil with mulch, and topsoil with clay. What is the question you are trying to answer?