What is effect size in quantitative research studies?
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At face value I’d have to say it’s the measured output of prescribed inputs into some defined process or system. Without the context I can’t be much more specific than that. I write research as a part of my job and any time I’ve described an effect size it’s a characterization of the outputs of some process, and the magnitudes of how those outputs change relative to the size of the outputs, for a given input modification.
Simple example: You’re driving 60mph and you have the magical ability to keep your foot perfectly still on the gas pedal. You’re driving on a perfectly flat road with zero wind. You turn on the A/C, which increases the torque demand on the motor by engaging the clutch in your A/C compressor. Now the car is losing a small fraction of horsepower to the A/C compressor instead of transmitting it through the drivetrain to the wheels to combat aero resistance and friction to propel your car forward. Increasing the losses means your vehicle speed will drop ever so slightly. The effect size would be a measure of the speed change due to turning on the A/C. Roll the windows down now. Air resistance will increase again and your speed will drop further. The effect size would be that change in speed. Leave for foot exactly where it is on your gas pedal but force the transmission to downshift a gear. You’ll slow rapidly to a new rate given the same throttle setting on your gas pedal. The effect size is your change in speed. You hit a small incline. Now you’re working against gravity and it’ll slow you down. Change in speed is the effect size. Take your foot off the gas and slam on the brakes. Your speed changes.
Lots of different things can result in changes in speed, depending on which input changed. The effect size is a good way to rank all the potential input changes to see which is most effective in altering your speed.
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