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Statistics without maths for psychology
Figure 7.5 Strong positive skew
Figure 7.6 Strong negative skew
When we have different numbers of participants in the two groups, taking a simple average
of the two variances might be misleading because the formula would give the two groups
equal weighting, when in fact one group might consist of more participants. In this case we
would use a weighted average. The weighted average for the sample (called the pooled variance
estimate) is used in order to obtain a more accurate estimate of the population variance.
If your data are extremely skewed and you have very small participant numbers, you will
need to consider a non-parametric test (see Chapter 16). This is because non-parametric tests
do not make assumptions about normality.
7.1.10 t-test for independent samples
Let’s use our example of the NOISE/NO NOISE experiment to go through the SPSS instructions
and output for a t-test for independent samples.