99% Rare Deviate Norm 5% Distribution 95% Standard Mean High Or Low 2Sds Typical 1%

Statistical infrequency

Behaviours which are statistically ...... (atypical) or ...... from the statistical average or ...... are classified as abnormal. Statistical norms are established using a normal ...... curve which illustrates the fact that most people, (approx ...... ) score between one and two ...... deviations above or below the ...... This leaves just over ...... of people who score unusually ...... (more than or less than ...... from the mean) and these people would be classed as abnormal because they are not ...... of most people. Schizophrenia is suffered by ...... of people, ..... do not have the condition and therefore by statistical standards it is rare and thus abnormal.

99% Rare deviate norm 5% distribution 95% standard mean high or low 2SDs typical 1%

Taking a statistical approach

First of all if is necessary to assess what is normal within a population. If we wanted to find out the statistical norm for happiness, we could ask people to complete a happiness questionnaire and calculate the average score (mean).

Many psychological variables (e.g. intelligence, aggression, extroversion) have what is known as ‘normal distribution’ meaning that most people score close to the mean, with an even amount of scores above and below, thus only a small minority score exceptionally high or exceptionally low. This can be shown on a ‘normal distribution’ or ‘bell curve’. We are going to find out whether our sample of happiness scores are normally distributed.

In terms of describing our data set not only can we say what the average score is, we can also say how the scores are distributed (dispersed) around the mean. The most sensitive ‘measure of dispersion’ is known as the standard deviation and it involves a relatively complex calculation where every score is subtracted from the mean. It tells how much on average the scores differ from the mean. When the standard deviation is relatively high, scores are more varied (there are greater individual difference between Pps), when the standard deviation is relatively low, scores are more tightly clustered around the mean (Pps are achieving more similar results).

Strength of the standard deviation: Because the standard deviation uses all the scores in a data set, it is the most powerful and sensitive measure of dispersion.

In a normally distributed sample:

·  68.26% of people will score between one standard deviation above or below the mean.

·  95.44% of people will score between two standard deviations above or below the mean.

·  99.74% of people will score between three standard deviations above or below the mean.

This means that anyone who scores more than two standard deviations above or below the mean is in a minority of about 5% and thus could be seen as statistically abnormal.