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Sometimes in psychology we study phenomenon that are subjective or vary across different people. For example, if I want to study "happiness" in children, I might consider happiness to be "# of laughs". However another researcher who is trying to replicate my work might consider happiness to be "time not spent crying". To help researchers understand exactly what is being measured, I would need to specify THIS- which definition that explains exactly what I was measuring when I said I measured happiness.
What is an OPERATIONAL DEFINITION?