Factors originating within a system or study that influence outcomes and are, in principle, part of what the researcher is studying or should be controlling. In experimental design, internal variables include the treatment itself, participant characteristics, measurement procedures, and all the elements the researcher deliberately manipulates or measures. However, internal variables also include unintended factors: participant expectations (placebo effects), researcher expectations (experimenter bias), measurement error, and all the ways the study itself shapes its own results. Distinguishing intended internal variables from confounding internal variables is the fundamental challenge of research design.
Internal Variables Example: "They thought they were measuring the drug's effect, but the internal variable of participant expectation—they all knew they were getting the real drug—confounded everything. The improvement might have been pure placebo."
by Dumu The Void March 12, 2026
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