How do positive control groups work?

A positive control in an experiment is a group that gets a treatment with a predictable outcome. It serves as a standard to compare against the experimental group, helping scientists ensure that any changes observed are due to the treatment being tested and not other factors. If the positive control produces the expected outcome, it provides assurance that the experimental conditions are suitable for detecting similar effects in the experimental group. For instance, if one is testing lettuce for bacteria they may use a positive control group (when the lettuce is contaminated). This is done by swabbing the leaves on the lettuce with a known colony of bacteria, transferring the sample onto a dish designed for bacterial growth, then allowing it to incubate. After incubation, researchers are able to observe and identify any bacterial colonies that have grown on the dish. If no growth is observed, it suggests that there might be an issue with the experimental setup hindering bacterial growth. Thus, by including a positive control, researchers can verify that the experimental setup is capable of detecting the desired effect.