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Unit 4 - Controlled Variable

In scientific investigations there are three types of variable - key independent variable (what you change), dependent variable (what you measure) and the controlled variables (factors you control to ensure they are the same throughout your experiment to make it fair). This GCSE Biology quiz is all about the third type.

Controlled variables make sure that your experiment is a fair test, but sometimes you may miss one of the control variables. When this happens the results and conclusion may not be fully reliable. At other times, some important variables may be outside of your control or be extremely difficult to control. In some cases, changes in a variable that have not been measured could actually be the cause of changes in the dependent variable. This can lead to a false correlation between the variables you studied but this is rare. Where you do identify a control variable outside of your control, you should at least try to monitor it. That way, you can probably spot if it has had an effect on the results even if you can't measure it.

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Experiments in biology are actually very difficult to control completely. Take the study of "Does adding fertiliser make a plant grow bigger?". For a start, there are many different fertilisers so the results could differ from one to another. You would need to do many different experiments. The plants you use is a variable that needs to be controlled so you use the same type of plant in all of your tests. But genetics of each plant should also be controlled by using clones. Even if you did use cloned plants, there are other factors that should be controlled to ensure a fair test, like using the same soil. same size pot, same amount of light and temperature and some other factors too.

The conclusions that you draw from any biological experiment should show that you understand this and suggest some of the places that inaccuracy could have come into your experiment through no fault of your own. To overcome this, professional scientists repeat experiments many, many times so that they can 'average' the results. This helps to reduce any inaccuracies caused by the controlled variables.

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  1. In an ecology experiment, we must always use quadrats of the same...
    Studies involving quadrats require calculations made on area so using quadrats of different sizes would mean each sample would be taken over a different area. The control variable of area would therefore not have been controlled properly and could give invalid results
  2. We control the controlled variables by keeping them...
    Controlled variables must be kept the same
  3. If we fail to control variables, the results from the experiment may not be...
    The results will certainly be real but answers 2 and 3 depend on the person carrying out the experiment and not on controlled variables
  4. Unless we keep the controlled variables the same, the investigation will not be...
    A fair test is where the controlled variables are kept constant
  5. A fair test is one where everything is the kept same except for the...
    Only the independent variable is changed although the dependent variable will also be changing. The controlled variables remain constant
  6. In an experiment on the effect of temperature on germination of cress seeds, which of the following would be controlled variables?
    Temperature is the independent variable in this experiment so it cannot be a controlled variable. Any answer to this question containing the word temperature must therefore be wrong
  7. During a drug trial, which of the following are controlled variables?
    Lifestyle could have an effect on the results but is very complex and includes many other variables. It is for that reason that it would not really be regarded as a variable but more as a group of variables
  8. What is a variable?
    At the start of an investigation, you should list as many variables as you can
  9. In an experiment investigating the effect of acid concentration, which of these is a controlled variable?
    The container the acid is in and the time of day are nothing to do with the concentration of an acid
  10. Control variables are...
    Control variables should be kept as constant as possible during your experiment

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