characteristics of distributions- standard deviation

• when data is normally distributed, 68% of scores in the data set fall within one standard deviation of the mean, and 95% of scores are within two standard deviations of the mean
• statistical infrequency- how far someone's score is from the mean score (measure of SD) is one way of defining abnormal behaviour. One of the criteria for a diagnosis of intellectual disability (ID) is an IQ 2 SD below the mean, an IQ of 70
• how to work out standard deviation- mean of numbers, subtract it from each of the numbers you have at the beginning, will have a mix of minuses and positives and 0s, next, square your new numbers (will all be positives), get total of all of squared numbers, then work out mean again, then find square root of this