Exploring data: graphs and numerical summaries
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# 2.6 Body weights and brain weights for animals

The next data set comprises average body and brain weights for 28 kinds of animal, some of them extinct. The data are given in Table 6.

## Table 6 Average body and brain weights for animals

Species | Body weight (kg) | Brain weight (g)
--- | --- | ---
Mountain Beaver | 1.350 | 8.100
Cow | 465.000 | 423.000
Grey Wolf | 36.330 | 119.500
Goat | 27.660 | 115.000
Guinea Pig | 1.040 | 5.500
Diplodocus | 11700.000 | 50.000
Asian Elephant | 2547.000 | 4603.000
Donkey | 187.100 | 419.000
Horse | 521.000 | 655.000
Potar Monkey | 10.000 | 115.000
Cat | 3.300 | 25.600
Giraffe | 529.000 | 680.000
Gorilla | 207.000 | 406.000
Human | 62.000 | 1320.000
African Elephant | 6654.000 | 5712.000
Triceratops | 9400.000 | 70.000
Rhesus Monkey | 6.800 | 179.000
Kangaroo | 35.000 | 56.000
Hamster | 0.120 | 1.000
Mouse | 0.023 | 0.400
Rabbit | 2.500 | 12.100
Sheep | 55.500 | 175.000
Jaguar | 100.000 | 157.000
Chimpanzee | 52.160 | 440.000
Brachiosaurus | 87000.000 | 154.500
Rat | 0.280 | 1.900
Mole | 0.122 | 3.000
Pig | 192.000 | 180.000

(Jerison, H.J. (1973) Evolution the brain and intelligence. Academic Press, New York.)

These data raise interesting questions about their collection and the use of the word ‘average’. Presumably some estimates may be based on very small samples, while others may be more precise. On what sampling experiment are the figures for Diplodocus, Triceratops and other extinct animals based? The three-decimal-place ‘accuracy’ given throughout the table here is extraordinary (and certainly needs justification).

Putting these concerns to one side for the moment, it would seem obvious that the two variables, body weight and brain weight, are linked. But what is the relationship between them and how strong is it? Can the strength of the relationship be measured? Is a larger brain really required to govern a larger body? These data give rise to a common problem in data analysis which experienced practical analysts would notice as soon as they look at such data. Can you identify the difficulty? Later, when we plot these data, you will see it immediately.

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