Distribution curves

• Normal distribution

  shown in this line graph, a normal distribution is sometimes known as a ‘bell curve’. Many human characteristics, when measured, show a normal distribution i.e. the average is most common, the extremes very rare. The line down the centre of the graph represents the mean, median and mode. Standard deviations can also be plotted on a normal distribution – moving one standard deviation either side of the mean includes 68% of a data set, two standard deviations includes 95% and three over 99%.
• Skewed distribution

  do not follow a normal distribution. In a negative skew, there is a higher frequency of higher scores but some very low outliers which skew the distribution. In a positive skew, there is a higher frequency of lower scores but some very high outliers which skew the distribution. Skewed distributions give measures of central tendency which are more distinct from each other.