Experimental Design

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Experimental Design

There are many things we have to consider before we begin an experiment. You must have a good understanding of what you are investigating, and what you expect in your results.

Experimental Hypotheses

A hypothesis is a proposed explanation of an observation based on limited evidence.

An investigations purpose is to test a hypothesis, to see whether it is true or not. The hypothesis is used to see if there is any relationship between two variables. See an example below:

 Observation Dogs with big tails run fast. Hypothesis Having a big tail causes a dog to be able to run faster. Investigation Investigating if there is a relationship between a dogs speed and its tail size.

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Experimental Evidence

The results of your investigation/experiment will be evidence. How useful this evidence is in supporting your hypothesis depends on how you conducted your experiment.

• Repeatability: This means that if the same person was to do the same experiment using exactly the same methods, they would produce the same (or similar) results.
• Reproducibility: This means that if someone else was to do the same experiment, or using a different method, they would produce the same (or similar) results.

The most reliable results are the ones that are both reproducible and repeatable. Valid results also give a satisfactory answer to the original hypothesis.

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Variables

In an investigation, there are many variables at play. Usually you will be changing one variable and observing it’s affect on another. There are names for these different variables:

• Independent Variable: This is the variable you are changing.
• Dependent Variable: This is the variable you are measuring.
• Control Variables: These are the variables that you are keeping the same.

See some examples below of different investigations and their variables.

 Investigating the effect of pH on the rate of reaction Independent Variable pH Dependent Variable Rate of reaction Control Variables Temperature, type of enzyme, mass of enzyme etc

 Investigating the relationship between force and extension Independent Variable Mass Dependent Variable Extension of spring Control Variables Spring used, temperature of the room etc.
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Experimental Design Example Questions

Question 1: Decide which of the following key words match the sentences.

[3 marks]

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Question 2: Define repeatability and reproducibility.

[2 marks]

GCSE AQA
• Repeatability: If the same person was to do the same experiment using exactly the same methods, they would produce the same (or similar) results.
• Reproducibility: If someone else was to do the same experiment, or using a different method, they would produce the same (or similar) results.

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Question 3: Define the three types of variable in an experiment.

[3 marks]

GCSE AQA
• The independent variable is the variable being changed.
• The dependent variable is the variable being measured.
• Control variables are the variables that stay the same.