When a researcher inadvertently creates a condition with a fundamentally different type of participant than another condition, this can create a –. You can combat this by arbitrarily assigning participants to each level, known as –. In some cases, especially with smaller sample sizes, researchers will assign participants to groups so that each group has a similar makeup for a particular attribute they may be concerned about. This is known as creating –.

Selection effect occurs when there's an association or connection between health outcome and exposure is generally distinct for the people who have completed a specific study as compared to the people who fall in the target population.

Random assignment is also referred as random placement and is described as one of the experimental techniques in order to assign animal subjects or human participants in various groups in a particular experiment i.e, control or treatment group utilizing randomization.

Matched groups are described as a technique in various research design whereby a subject or participant in a specific experimental group that is being exposed to "manipulation" and is compared towards an outcome variable related to particular participant in "control group" and are being similar in a few important ways yet not receiving any sort of manipulation.