AP® STATISTICS

STATISTICS

2011 SCORING GUIDELINES

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Question 3

Intent of Question

The primary goals of this question were to assess students’ ability to (1) describe a process for

implementing cluster sampling; (2) describe a statistical advantage of stratified sampling over cluster

sampling in a particular situation.

Solution

Part (a):

The following two-step process can be used to select the eight apartments.

Step 1: Generate a random integer between 1 and 9, inclusive, using a calculator, a computer program,

or a table o

f random digits. Select all four apartments on the floor corresponding to the selected

integer.

Step 2: Generate another random integer between 1 and 9, inclusive. If the generated integer is the

same as the integer gene

rated in step 1, continue generating random integers between 1 and 9

until a different integer appears. Again select all four apartments on the floor corresponding to

the second selected integer.

The cluster sample consists of the eight apartments on the two randomly selected floors.

Part (b):

Because the amount of wear on the carpets in apartments with children could be different from the

wear on the carpets in ap

artments without children, it would be advantageous to have apartments

with children represented in the sample. The cluster sampling procedure in part (a) could produce a

sample with no children in the selected apartments; for example, a cluster sample of the apartments on

the third and sixth floors would consist entirely of apartments with no children. Stratified random

sampling, where the two strata are apartments with children and apartments without children,

guarantees a sample that includes apartments with and without children, which, in turn, would yield

sample data that are representative of both types of apartments.

Scoring

Parts (a) and (b) are scored as essentially correct (E), partially correct (P) or incorrect (I).

Part (a) is scored as follows:

Essentially correct (E) if the response correctly addresses the following two components:

1. Indication that two floors are randomly

selected, with all four apartments on each of the

selected floors forming the sample (or that the entire floors should be carpeted).

2. Description of a valid r

andom sampling procedure for selecting two floors that could be

implemented after reading the response (so that two knowledgeable statistics users would use

the same method to select the floors).

Partially correct (P) if the response includes exactly one of the two components listed above.

STATISTICS

2011 SCORING GUIDELINES

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Question 3 (continued)

Incorrect (I) if the response includes neither of the two components listed above OR the response does

not involve t

aking a random sample of two floors out of the nine.

Note: Some possible errors in component 2 include the following:

• Using 10 random digits rather than nine

• Failing to explicitly deal with the issue of potentially repeated random numbers

Part (b) is scored as follows:

Essentially correct (E) if the response indicates the following two components:

1. The amount of carpet wear could be different for apartments with and without children.

2. The stratified random sample ensures that some apartments with children will be selected.

Partially correct (P) if the response includes exactly one of the two components listed above.

Incorrect (I) if the response fails to meet the criteria for E or P.

Notes

• If the response in part (b) says that this stratified sampling method guarantees proportional

representation of a

partments with and without children, then the second component is satisfied.

• If the sampling procedure in part (a) divides the floors into two groups — those that have

apartments with children and those that do not

(“prestratification”) — and then selects one floor

from each group, score part (b) based on the degree to which a statistical advantage of the

stratified sampling in part (b) is addressed.

4 Complete Response

Both parts essentially cor

rect

3 Substantial Response

One part essentially correct and one part partial

ly correct

2 Developing Response

One part essentially correct and one part incorrect

Two parts partially correct

1 Minimal Response

One part par

tially correct and one

part incorrect

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STATISTICS

2011 SCORING COMMENTARY

Question 3

Overview

The primary goals

of this question were to assess students’ ability to (1) describe a process for

implementing cluster sampling; (2) describe a statistical advantage of stratified sampling over cluster

sampling in a particular situation.

Sample: 3A

Score:

In part (a) the st

udent assigns each floor a unique number, 1–9, and correctly describes the use of a

random number generator to select two floors, including what to do in the case of repeated random

numbers. The student then indicates that all eight apartments on the selected floors will be carpeted.

Part (a) was scored as essentially correct. In part (b) the student begins by stating that for cluster

sampling “it is possible that no apartments with kids would be selected,” and gives the combinations of

two floors for which this statement is true. The student argues that this is “bad … because kids tend to

wear out carpets more than adults,” and provides a consequence of such a sample for the owner. The

student then states that a “stratified sample would work better” because the sample will contain

“apartments with and without children.” The strong response in this part was scored as essentially

correct. Because both parts were scored as essentially correct, the response earned a score of 4.

Sample: 3B

Score:

In part (a) the st

udent suggests that one “could randomly select 2 floors to carpet.” Although this does

not explicitly state that eight apartments are to be carpeted, the idea of carpeting entire floors is

sufficient to indicate that four apartments on each floor will be carpeted. However, the student makes

no attempt to implement a valid random sampling procedure for selecting the two floors. Part (a) was

scored as partially correct. In part (b) the student indicates that the “advantage to this [the stratified

sample] would be to see if kids play a big role in the carpets [sic] wear,” as opposed to the cluster

sample in which “you might not get any kids.” The communication is weak, but the student does

address the difference in carpet wear between the apartments with and without children and also

indicates that a cluster sample might not include apartments with children. Part (b) was scored as

essentially correct. Because one part was scored as essentially correct and one part was scored as

partially correct, the response earned a score of 3.

Sample: 3C

Score:

In part (a) the student describes a non-cluster sample and does not describe a random sample of two

oors from the nine. Part (a) was scored as incorrect. In part (b) the student indicates in the first two

sentences that a stratified sample will have apartments with and without children (as opposed to the

cluster sampling method) and that “you get to see how the carpet wears” in both types of apartments.

This is sufficient to answer the question; the remaining sentences were considered extraneous and did

not detract from the response. Part (b) was scored as essentially correct. Because one part was scored as

essentially correct and one part was scored as incorrect, the response earned a score of 2.

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