Short answers, pulled from the story.
A scientific control is an element of an experiment or observation designed to minimize the influence of variables other than the independent variable under investigation. This process reduces the risk of confounding by ensuring that only the factor being tested changes while all others remain constant.
Negative and positive controls are the simplest types of control found in many different types of experiments. When both succeed, they usually eliminate most potential confounding variables because the experiment produces a negative result when expected and a positive result when expected.
Lousdal et al examined the effect of screening participation on death from breast cancer and hypothesized that screening participants are healthier than non-participants. They used proxies for better health as negative-control outcomes to test if exposure was associated with outcome and determine if causally influence exists.
If the study design is valid, there should be no statistical association between the Negative Control Outcome and the treatment. An association between them suggests that the design is invalid and the effect of the study on outcome is non-identifiable.
In randomization, the groups that receive different experimental treatments are determined randomly so that differences are distributed equally. This method mitigates the effect of variations in soil composition on yield or other environmental factors affecting crop yield.