The Art of Statistics

the overall rate for all births in the US in 2013 was 8%, while the rate in black women was 13%, a notable difference between races.

shows that we would expect 1.7% of babies in this group to be low birth weight – in fact the actual number was 14,170 (1.3%), in close agreement with the prediction from the normal curve.

medical rather than statistical reasons, babies below 2,500 g are considered ‘low birth weight’, and those below 1,500 g ‘very low birth weight’.

11th percentile, which means that 11% of full-term babies born to non-Hispanic white women will weigh less – Figure 3.2(d) shows this 11% as a light grey shaded area.

the 25th and 75th percentiles (3,791 g) are known as the quartiles, and the distance between them (624 g), known as the inter-quartile range, is a measure of the spread of the distribution.

around 1.2 standard deviations below the mean – this is also known as her Z-score, which simply measures how many standard deviations a data-point is from the mean.

95% of
the population will be contained in the interval given by the mean ± two standard deviations, and 99.8% in the central ± three standard deviations.

the difference is that terms such as mean and standard deviation are known as statistics when describing a set of data, and parameters when describing a population.

Other, less natural phenomena may have population distributions that are distinctly non-normal and often feature a long right-hand-tail, income being a classic example.

The smooth normal curve and the histogram are gratifyingly close, and other complex traits such as height and cognitive skills also have approximately normal population distributions.