There many factors in good experimental design. Books are written on the subject. You will see terms like,
Randomized control trial (RCT) - In a clinical trial, people are randomly assigned to a control group who receives a placebo and the group which receives the treatment under investigation.

Double Blind - Neither the subjects or experimenters know which group is getting the treatment under test.


Books:

Links:
Experimental Design
Correlation

last updated 20 Mar 2017