For example, suppose you wanted to look at the relationship between higher income and longer lifespans. A high income does not act directly on someone’s lifespan in a positive way, but it may allow access to better nutrition and health care. This better nutrition and health care, in turn, affect the life span of the person, and so these are intermediate variables.
The intermediate variable is sometimes also called the intervening variable, the mediating variable, or the intermediary variable.
The Intermediate Variable and Full and Partial Mediation
Coming to an understanding of how a causal relationship involves an intermediary variable is called finding a mediation model, which may take different forms. A mediating variable can account for all of the causal links between two variables, or it can only be responsible for a small part. In some situations, there may be more than one mediating variable.
If the independent variable only affects the dependent variable through an intermediary variable, it’s called full mediation. If you drew your mediation model as in the image above, you would want to leave out path C. The only significant relationship is through the A/B path.
If the intervening variable is responsible for just part of the relationship, it’s called partial mediation. In this case, path A/B and path C are both significant, and disabling the mediator variable will not stop the causal process.
Difference between moderating and intermediate variable
Although they sound similar, moderating variables are conceptually different from mediating (intermediate) variables. The difference is very subtle; Depending on your question, some variables can be both a moderator and a mediator .
- Moderators answer the question “when?” or “under what conditions?”. Moderators can strengthen, weaken, or reverse a relationship. For example, confidence in your ability to perform a task moderates the relationship between task importance and the amount of anxiety felt .
- Mediators answer “how?” or “why?”. For example, people with higher incomes tend to live longer because of a mediator variable: access to better health care.
 What are mediation and moderation? Retrieved June 30, 2022 from:https://ademos.people.uic.edu/Chapter14.html#1_what_are_mediation_and_moderation
 Nie, Y., Lau, S., & Liau, A. K. (2011). Role of academic self-efficacy in moderating the relation between task importance and test anxiety. Learning and Individual Differences, 21(6), 736-741.
Types of Variables. Y520 Fall 2002 Course Notes: Strategies for Educational Inquiry. Indiana University. Retrieved from http://www.indiana.edu/~educy520/sec5982/week_2/variable_types.pdf on June 22, 2019.
Nemerever, Zoe. Visualizing Poli 30. Retrieved from https://pages.ucsd.edu/~tfeenstr/teaching/Winter2018/Visualizing_POLI30.pdf on June 22, 2019.