Why is a control group nessasary in a scientific investigation?

Answer 1

The concept of a control group is commonly used in the context of pharmaceutical research but can apply equally in other areas where you are testing the efficacy of something, for instance a chemical like a pesticide or fertiliser.

Control groups of all kinds establish a baseline for measuring an effect. You can use a zero baseline where the control group is not subject to any process to measure the absolute change under the new regime. Alternatively you can use a relative baseline where the control group is subject to an existing process to measure the relative benefits of the new process.