Besides using these principles of experimental design in your own on-farm research, you can use them to evaluate the claims of consultants, salespeople, and neighbors. Someone may try to sell you on a practice or a product based on positive results elsewhere. You can ask questions about how well the results were replicated, and whether the conclusions being drawn are reasonable or likely to be the result of conditions peculiar to another farm or situation. If the results weren't based on a replicated trial and repeated from one year to the next, it doesn't necessarily mean that they're wrong. It means you should use caution and preferably experiment on a small scale with some replication before you believe the results.
Variables can be either qualitative or quantitative.
For example, if compost is applied to an orchard at ten different rates, that is a quantitative variable. If one is comparing conventional to organic production systems, this variable is qualitative. Different species of cover crops are also distinguished qualitatively. Most dependent variables of interest to farmers can be measured quantitatively. For example, yield, cull rates, pest incidence, insect or earthworm densities, and soil organic matter can usually be assessed as quantitative variables.