HUMAN BIOLOGY

12 Chapter 1

ExpLorEon yoUr oWn

As you read in Section 1.4, having a sample of test subjects or observations that is too small can skew the

results of experiments. This phenomenon is called sampling error. To demonstrate this for yourself, all you need is a partner, a blind-

fold, and a jar containing beans of different colors—jelly beans will do just fine (Figure 1.13). Have your partner stay outside the room while

you combine 120 beans of one color with 280 beans of the other color in a bowl. This will give you a ratio of 30 to 70 percent. With the

bowl hidden, blindfold your partner; then ask him or her to pick one bean from the mix. Hide the bowl again and instruct your friend to

remove the blindfold and tell you what color beans are in the bowl, based on this limited sample. The logical answer is that all the beans

are the color of the one selected.

Next repeat the trial, but this time ask your partner to select 50 beans from the bowl. Does this larger sample more closely approxi-

mate the actual ratio of beans in the bowl? You can do several more trials if you have time. Do your results support the idea that a larger

sample size more closely reflects the actual color ratio of beans?

Figure 1.13 Animated! Here’s one way you can demonstrate sampling error.

A Natalie, blindfolded,

randomly plucks a jelly

bean from a jar of 120

green and 280 black jelly

beans, a ratio of 30 to

70 percent.

B The jar is hidden

before she removes her

blindfold. She observes

a single green jelly bean

in her hand and assumes

the jar holds only green

jelly beans.

C Still blindfolded,

Natalie randomly picks

50 jelly beans from the

jar and ends up with 10

green and 40 black ones.

D The larger sample

leads her to assume

one-fifth of the jar’s jelly

beans are green and

four-fifths are black (a

ratio of 20 to 80 percent).

Her larger sample more

closely approximates the

jar’s green-to-black ratio.

The more times Natalie

repeats the sampling, the

greater the chance she

will come close to know-

ing the actual ratio.

All Photos: © Cengage Learning/Gary Head

A C

B D

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