Exploring data: Graphs and numerical summaries
Exploring data: Graphs and numerical summaries

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Exploring data: Graphs and numerical summaries

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

SpeciesBody weight (kg)Brain weight (g)
Mountain Beaver1.3508.100
Cow465.000423.000
Grey Wolf36.330119.500
Goat27.660115.000
Guinea Pig1.0405.500
Diplodocus11700.00050.000
Asian Elephant2547.0004603.000
Donkey187.100419.000
Horse521.000655.000
Potar Monkey10.000115.000
Cat3.30025.600
Giraffe529.000680.000
Gorilla207.000406.000
Human62.0001320.000
African Elephant6654.0005712.000
Triceratops9400.00070.000
Rhesus Monkey6.800179.000
Kangaroo35.00056.000
Hamster0.1201.000
Mouse0.0230.400
Rabbit2.50012.100
Sheep55.500175.000
Jaguar100.000157.000
Chimpanzee52.160440.000
Brachiosaurus87000.000154.500
Rat0.2801.900
Mole0.1223.000
Pig192.000180.000

(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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