What is an “operational definition”?
It is important that a research knows exactly what their independent and dependent variables are. Thus, they need to have an operational definition. Operational definitions are useful when the DV is quite subjective. For instance, in the gum “experiment” the researchers were measuring the perception of the twins. Perception is such a broad term and if someone was to try to replicate (copy) this experiment they would not know how to measure “perception”. So an operational definition is needed whereby “perception” is defined as it is measured in the study.
To use my gardening study. The IV is the presence or absence of fertilizer and the DV is the garden growth. But garden growth is too vague and it’s not quantifiable. This is another purpose of operational definitions: to quantify otherwise qualitative behaviour (read more about the differences between qualitative and quantitative data below). So the operational definition of my DV could be: the mean length (in cm) of growth that occurs in each plant when measured from the base to the tallest tip. There are lots of ways to measure “growth” but this is how I’ve decided to measure it in my garden experiment.
IQ, memory, attraction, violence, conformity…these are all behaviors that are not easily quantifiable and open to interpretation. This is why experiments must use operational definitions.