Extraneous variable

Extraneous variables are variables other than the independent variable that may bear any effect on the behavior of the subject being studied.

Classification

Extraneous variables are often classified into three types:

1. Subject variables, which are the characteristics of the individuals being studied that might affect their actions. These variables include age, gender, health status, mood, background, etc.
2. Experimental variables are characteristics of the persons conducting the experiment which might influence how a person behaves. Gender, the presence of racial discrimination, language, or other factors may qualify as such variables.

These are changes that occur within the subjects during the passage of time. Variables such as physical growth, aging, hunger, etc. change over time and tend to be confounding variables to the experiment. For example, if you put a long span between the pre-testing and the post-testing of infants during your study of memory, it will not be internally valid for the reason that infants brain development is high, and its brain may have developed enough to have an effect on the post-test, thus showing that there is an increase in the memory capabilities of the infant, notwithstanding the fact that it has grown over time.

Instrumentation

This is a threat to validity due to some misdemeanors on the part of the experimenter or checker, it has nothing to do with participants.

Selection

Improper assignment of test units to treatment conditions. This problem can be solved by random assignment of test units to treatment conditions.

Statistical regression

Similar to testing carryover, except that regression is the phenomenon that extreme scores change more from pretest to posttest than do average scores.

Attrition or experimental mortality

A subject quits the experiment while the experiment is in progress.