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• Pair up. One of you toss a coin 10 times. The other record on a small piece of paper how many times it comes up ‘heads’. When you have recorded 10 results, give the paper to me. Repeat until I ask you to stop.

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Today’s session

You are learning about... You are learning to...• Probability• Probabilistic research

• Make judgements about significance

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• Unlike much of what you are used to in the physical sciences, judgements in psychology are often based on probability

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• When you toss a coin, what is the probability it will come up ‘heads’?

• If you have thrown nine heads in a row, what happens to the probability that your next toss will be ‘heads’?

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• If we plot the results from your earlier coin tosses on the graph below, what shape will we get?

O heads; 10 tails

5 heads; 5 tails

10 heads; 0 tails

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Normal distribution curve

O heads; 10 tails

5 heads; 5 tails

10 heads; 0 tails

Likely outcomes

Unlikely outcomes

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• Does your partner have telekinetic powers?– Decide who will be the researcher and who will be

the participant.– Participant will use the power of their mind to will

the coin to come up ‘heads’

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• If your participant’s coin came up heads, does that mean she has telekinetic powers?

• So how can we use coin tosses to test if she does?

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O heads; 10 tails

5 heads; 5 tails

10 heads; 0 tails

Likely to be caused by chance

Unlikely to be caused by

chance; perhaps caused by

something else

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• How many times out of ten must the participant’s coin come up heads before we accept that the result was not just due to chance?

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• Did any participant meet or exceed the criterion?– Does this (or would this) prove that she had

telekinetic powers?– What would we do to check?

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Significance

• In psychological research we judge the importance of results by comparing them with what is likely to happen by chance.

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• Everyone choose a number between 1 and 10 and write it down

• We’ll divide the class arbitrarily in half and compare the numbers they have chosen

• Do we expect that the two sets of numbers will be very different?

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• The two sets of numbers will not be identical; they will be different. However:– The difference may be due to chance– The difference may be due to something else

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• Let’s repeat the test. This time, read what’s on the card before choosing your number.

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Significance

• The more consistent the difference between the two sets of numbers, the smaller the probability that the difference was caused by chance.

• However, we have to decide how unlikely a ‘chance result’ has to be before we will accept that the difference was caused by something else

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Significance

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