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AS Statistics Revision
Statistics 1 Chapter 4
Methods of summarising data (dispersion)
Key points to remember
Range = the value obtained when the smallest observation in
data set is subtracted from the largest one.
Interquartile range (IQR) = the value obtained when the
quartile is subtracted from the upper quartile.
IQR = Q3 Q1
Outlier = an extreme value which does not fit into the main
of the data. Any rules to identify outliers will be specified in
S1 question paper. The usual equation tends to be either
For the lower outlier limit = LQ (1.5 x IQR)
For the upper outlier limit = UQ + (1.5 x IQR)
Variance = the variance of a population of observations. This
the standard deviation squared.
i.e. (fx² ÷ n) x²
Standard deviation = (fx² ÷ n) x²
Skewness = an indicator of the shape of a distribution.
Symmetrical: mode = median = mean
Positive Skew: mode < median < mean
or Q2 Q1 < Q3 Q2
Negative Skew: mean < median < mode
or Q2 Q1 > Q3 Q2