Exploring data: Graphs and numerical summaries

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# 1.1.7 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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