Practical considerations when choosing variables

Dependent and independent variables

Sometimes choosing which variables are independent, dependent and control/controlled is straightforward based on the aim of your experiment. Other times, it can be fairly arbitrary.

If there is no conceptual advantage in choosing certain parameters to be the dependent, independent or control/controlled, the operation of the equipment could provide the answer for which variable is which based on practical consideration.

If you need to set a value to exactly the same conditions many times this may be easier to do, or more precisely controlled, with one piece of equipment or instrumentation than another. If a parameter can be set easily or can be set with a high precision, it is best to make this the independent variable.

Consider this experiment to determine the relationship between the pushing force from a jet of water and flow rate of that water. The flow rate of the jet is adjusted by turning a tap and the amount of force is determined for adjusting the amount of mass placed on a platform.

IoJ.mp4

In this video we consider two options:

We could set the flow rate as the independent variable using the tap and measure the amount of mass as the dependent variable.
Alternatively, we could set the amount of mass as the independent variable and measure the flow rate as the dependent variable.

How to choose controlled variable

If you are dealing with a system where you want to investigate the influence of 2 factors on the parameter you are measuring, how do you determine which variable is the independent and which is the controlled? Once again, if there is no clear preference from the aims of the experiment, then practical considerations can help make the decision. If it is easier or more precise to set the value of one parameter over the other, this should be chosen as the controlled variable.

Let's think about the pump example given in the previous section. The efficiency, which we want to measure, is influenced by the pump speed and the flow rate, so it isn’t clear which should be the controlled variable and which the independent variable. But let’s consider how the equipment might operate.

As an example, the pump's speed is set with a control system where the desired speed is input into a digital display and the water flow rate is set by adjusting the opening of a tap and viewing the measurement on a meter. In this example system, the flow rate will be altered by changing the pumps speed. If the flow rate is set as the controlled variable and the pumps speed is varied as the independent variable, the tap will need to be adjusted for every change of setting of the pump speed, in order to hold flow rate constant.

Maintaining a controlled variable of pump speed, by typing in a specific value into a control system, is much easier than incrementally opening and closing a tap until a target value is displayed on the instrumentation. So, for this example, it would be sensible to set the speed as the controlled variable and the flow rate as the independent variable.

Top Tip! Record justification of decisions made when choosing variables, as this will be useful information to include when writing up findings.

Summary: If the aim or concept of the experiment doesn't make clear which parameters to choose and the dependent or independent variable or the independent or controlled variable, the practical operation of the equipment could provide the means to make a decision. This is because setting or measuring parameters can be tricky, imprecise or time consuming. There are usually only a limited number of options and so it is wise, during an experimental design, to consider all options, determine the most sensible and appropriately record the justification for your decision.

Next >>

Video transcript

In this experiment a jet of water is fired onto a surface and the force is measured. We want to find out the relationship between the flow rate of the jet and the force the jet creates on the surface. The flow rate of the jet is adjusted by turning a tap and the amount of force is determined for adjusting the amount of mass placed on a platform. When the gravitational force from the mass equals the force generated by the jet, the platform moves to a known position.

For each data point in the experiment, we could set the flow rate using the tap and measure the amount of mass on the platform by adjusting the amount of mass until the platform reaches the known position. In this case the flow rate would be the independent variable and the amount of mass would be the dependent variable. Alternatively, we could set the amount of mass on the platform and measure the flow rate by adjusting the opening of the tap until the platform reaches the known position. In this case the amount of mass would be the independent variable and the flow rate would be the dependent variable.

From a theoretical perspective, there is no advantage in one over the other. But from a practical perspective it is much easier to adjust the flow rate with the tap than it is the mass on the platform. If I set the flow rate and adjust the mass, it is tricky to add and remove masses of different weights until the platform returns to the known position.However, if I set the mass, I can easily adjust the flow rate with the tap until the platform returns to the known position.

Report abuse