Many characteristics that show continuous variation, such as height or leaf length, are affected by many genes and by the environment. When these are measured in a large sample and plotted as a frequency histogram, the data often form a normal distribution.A normal distribution curve is bell-shaped and symmetrical about the mean. Most values lie close to the mean and fewer values lie towards the extremes.In a normal distribution the mean, median and mode are all the same value, at the peak of the curve.The spread of the data is described by the standard deviation. About 68% of values lie within one standard deviation of the mean and about 95% lie within two standard deviations of the mean.Two normal distributions can have the same mean but different spreads. A wider, flatter curve has a larger standard deviation.
If a distribution is not symmetrical it is said to be skewed, and the mean, median and mode will be different.
If a distribution is not symmetrical it is said to be skewed, and the mean, median and mode will be different.