The Experimental Method:
(Independent & Dependent Variables)

 

How to choose the Levels of an Independent Variable:

1. Cover as much of the range as practical:

You never know what may happen in another time frame.

Example: Testing vision after dark adaptation. If you only kept people in the dark for 5 minutes and not 15, you would miss the switch from cones to rods.

2. Have enough values tested not to miss any important relationships?

Example:

The effects of arousal on performance (the Yerkes-Dodson function). Too little arousal leads to a lower level performance. Too much arousal leads to a low level of performance. Middle levels of arousal leads to best performance.

3. Pay attention to the underlying scale of the IV when picking values.

For Example: If you want to increase the loudness of a sound to see if it increases arousal (as in a Yerkes-Dodson experiment), you need to know how the logarithmic decibel scale works.

Change in dB

Change in sound energy

3 dB increase

Sound energy doubled

10 dB decrease

Sound energy decreased by factor of 10

20 dB decrease

Sound energy decreased by factor of 100

Therefore, when you want to make a sound twice as loud - what do you change (better take that Sensation and Perception class)?

 

Bottom Line:

You manipulate the Independent Variable (IV) in an experiment in most cases. Sometimes you have to select subjects with appropriate characteristics.

You do this because you can't manipulate the factor (ex. Gender).

You hope the manipulation of the IV will affect the Dependent Variable (DV). The DV is the variable you measure.

You have to watch out when you manipulate the IV for confounds and the ranges of IV values you choose.

It's really an easy idea.

 

Change the IV - Measure the DV