What is the Dependent Variable?

To make it easy to do the experiment, we only normally measure ONE THING the IV affects, or changes. 

The thing we measure is called the Dependent Variable, and we can call it the DV.

We can measure the DV directly (like ‘length of the fish’) or indirectly (like ‘speed of toy car going down a ramp’). Both these examples are DVs, but indirect measurements may need some extra steps to calculate them, but these will normally be part of the Results Table, you just need to pay a bit more attention to how you research and write about indirectly measured DVs in the lab report. 

The Variables table can really help you understand the role of the DV and the equipment you will use to measure it. 

So, how would you write this part of the table out as a sentence?

The dependent variable is the length of the fish, measured in cm, using a ruler that has been submerged in the tank. 

You will notice two things: 

1. You don’t need to write the "independent variable is the 1 thing I change",

2. This might be a little tricky and you could easily get the measurement wrong.

Can you see how a fish swimming past a ruler and you trying to read the numbers at the mouth and tail might introduce error into the experiment? Any ideas how you might actually take these measurements? 

Obviously, the technique was probably to avoid removing the fish from the tank and holding it against a ruler, as this would break the rules regarding animals in experiments, but technically this is difficult. A solution would be to take a photo of the fish swimming past the ruler, and try to read the numbers at either end of the fish from the photo. 

The good scientists reading this will already be able to spot several problems with this solution, perhaps including how far the fish is away from the ruler and the angle of the camera when the photos are taken, but this doesn’t mean you can’t use this technique. 

If you didn’t think of any other problems with this, don’t worry, everyone can evaluate a method, it just takes practice—but the Evaluation section of a lab report can be where you get the best marks. This is because it really shows which students can think and write beyond the information they have found in their research. So, write down any problems you see with the method, and use them in the Evaluation section, or as part of the Control Variables.

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