Normal Distribution and Central Tendency

Key points

• • Data are summarised by a measure of central tendency (a typical value) and a measure of spread (how scattered the values are).
• • The mean is the arithmetic average, the median is the middle value when data are ordered, and the mode is the commonest value.
• • The median is better than the mean for skewed data, because a few very high or very low values pull the mean but not the median.
• • The normal (Gaussian) distribution is a symmetrical bell-shaped curve in which the mean, median and mode all coincide at the centre.
• • In a normal distribution, close to 95 percent of values lie within two standard deviations of the mean; many reference ranges are based on the central 95 percent of a healthy reference population.
• • The standard deviation measures the spread of the data around the mean, and a larger standard deviation means more scattered values.