The Control Variables (CVs)

This is the only row in the Variables Table that is plural, i.e. you need to write down more than one thing. These are things that could change the result of the experiment and make it an unfair test. 

If we were measuring ‘fish growth’ using a photo of a fish swimming past the ruler, there are lots of things that could give us incorrect or slightly different results each time. These small differences can be smoothed out by calculating the average of your DV measurements, but you need to show how you will keep some things the same, to make it a fair test and minimise these differences. 

There will be a minimum of 3 or 5 Control variables (or CVs) depending on what year you are in, so use the markscheme from the beginning of the experiment to make sure you control a sufficient number of variables. This will make the range of DV measurements you make smaller (and for older students this will be shown as your Standard Deviation [SD] or Error Bars). 

If you are working with someone else, or other people in the lab are working on the same or similar experiment, Brainstorm with them what things could change the results of the experiment, and write your best ideas into the Variables Table.

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

The control variables are the species of the fish Fantail), the amount of food (10 g/day), and how far the camera is away from the tank (10 cm). A box will be placed outside the tank, in line with the ruler inside the tank. A line will be drawn on the box 10 cm away from the glass, this will also make sure the photos are taken at the same angle each time.

You will notice two things: 

1. Even though there are multiple CVs and you have to write how you will control them, you still don’t need to write much here.

2. I sneaked an extra CV into the sentences.

If you think of another CV when you are planning, doing, or writing about the experiment, put it in the lab report! Just make sure it goes in the right place: if it is before/when you are doing the experiment it’s a CV, if it is after…….. Not a problem! Put it in the Evaluation section.

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