AP® STATISTICS

STATISTICS

2011 SCORING GUIDELINES (Form B)

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Question 5

Intent of Question

The primary goals of this question were to assess students’ ability to (1) identify and check appropriate

conditions for inference; (2) identify and carry out the appropriate inference procedure; (3) determine the

sample size necessary to meet certain specifications in planning a study.

Solution

Part (a):

Step 1: Identifies the appropriate confidence interval by name or formula and checks appropriate

conditions

One-sample (or large-sample) interval for p (the proportion of the vaccine-eligible people in the

United States who actually got vaccinated) or

ˆˆ

(1- )

Conditions: 1. Random sample

2. Large sample (

≥ 10 and

np(1-≥

) 10

)

The stem of the problem indicates that a random

sample of vaccine-eligible people was surveyed.

The number of successes (978 vaccine-eligible people who received the vaccine), and failures

(1,372 vaccine-eligible people who did not receive the vaccine), are both much larger than 10, so

the large-sample interval procedure can be used.

Step 2: Correct mechanics

Êˆ

978 0.41617(1-0.41617)

Á˜

± 2.57583

Ë¯

2,350 2,350

0.41617 ±¥2.57583 0.01017

0.41617 ± 0.02619

(

0.38998, 0.44236

)

Step 3: Interpretation

Based on the sample, we are 99 percent confident that the proportion of the vaccine-eligible people

in the United States who actually got vaccinated is between 0.39 and 0.44. Because 0.45 is not in

the 99 percent confidence interval, it is not a plausible value for the population proportion of

vaccine-eligible people who received the vaccine. In other words, the confidence interval is

inconsistent with the belief that 45 percent of those eligible got vaccinated.

STATISTICS

2011 SCORING GUIDELINES (Form B)

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Question 5 (continued)

Part (b):

The sample-size calculation uses 0.5 a

s the value of the proportion in order to provide the minimum

required sample size to guarantee that the resulting interval will have a margin of error no larger than

0.02.

(2.576)

(0.5)(0.5)

Êˆ

2.576

n ≥=

Á˜

=4,147.36

(0.02)

Ë¯

2(0.02)

Thus, a sample of at least 4,148 vaccine-eligible people should be taken in Canada.

Scoring

Each step in part (a) is scored as essentially correct (E), partially correct (P), or incorrect (I); and part (b) is

scored as essentially correct (E), partially correct (P), or incorrect (I).

Step 1 of part (a) is scored as follows:

Essentially correct (E) if the one-sample z-interval fo

r a proportion is identified (either by name or

formula) AND both conditions (of random sampling and sample size) are stated and checked.

Partially correct (P) if the response identifies the correct procedure BUT adequately addresses only one

of the two required conditions, OR if the response does not identify the correct procedure BUT

adequately addresses both required conditions.

Incorrect (I) for any of the following:

• The response identifies the correct procedure BUT does not adequately address either required

condition,

• The correct procedure is not identified AND at most one of the required conditions is

adequately addressed,

• An incorrect procedure is identified.

Notes

• If the formula is of the correct form, even if incorrect numbers appear in it, then the procedure may

be considered correctly identified.

• Stating only that np

and n(1- p

) both are greater than or equal to 10 is only a statement of the

sample size condition and is not sufficient for checking the condition. The response must use

specific values from the question to check the condition.

• If a response includes additional inappropriate conditions, such as n ≥ 30 or requiring a normal

population, then the response can earn no more than a P for this step. However, stating and

checking a condition about the size of the sample relative to the size of the population is not

required but is also not inappropriate.

• Any statement of hypotheses, definitions of parameters, statements of populations, etc. should be

considered extraneous. However, if these statements are included and incorrect, this should be

considered poor communication in terms of holistic scoring.

STATISTICS

2011 SCORING GUIDELINES (Form B)

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Question 5 (continued)

Step 2 of part (a) is scored as follows:

Essentially correct (E) if a 99 percent c

onfidence interval is correctly computed.

Partially correct (P) for any of the fol

lowing:

• If a correct method (confidence interval for a proportion) is used, BUT an incorrect critical z-

value or a t-value is used.

• 0.45 is used for the value of p

• There are errors in the calculation of the interval (unless such errors follow from an incorrect

procedure in step 1).

Incorrect (I) if an incorrect method is used, such as a t-interval for a population mean, OR

if the resulting interval is unreasonable, such as an interval with integer endpoints.

Step 3 of part (a) is scored as follows:

Essentially correct (E) if the response notes that 0.45 is not in th

e 99 percent confidence interval AND

states that this is evidence against the belief that 45 percent of vaccine-eligible people had received

flu-vaccine.

Partially correct (P) if a reasonable statem

ent about the belief that 45 percent of vaccine-eligible people

is made, in context, but there is no clear connection made to the confidence interval, OR a clear

connection to the confidence interval is made, but the response includes one or both of the following

two omissions:

The response is not in context.

The response does not mention the confidence level of 99 percent.

Incorrect (I) if the response fails to meet the criteria

for E or P.

Part (b) is scored as follows:

Essentially correct (E) if an appropriate sample size is calculated and supporting work is shown.

Partially

correct (P) if supporting work

is shown, BUT the response includes one or both of the following

errors:

0.41617 (the sample proportion) or 0.45 is used instead of 0.5.

An incorrect critical z-value is used — unless the same incorrect value was used in part (a).

Incorrect (I) if the response fails to meet the criteria

for E or P.

Notes

• In this situation, the formula for margin of error used to compute sample size is only an

approximation of the margin of error. Because of this, we will not insist that the computed sample

size be rounded up; that is, 4,147 is scored as E, as long as supporting work is shown.

• If the critical value of 2.575 is used, then the sample size should be

n ≥ 4,144.14

n = 4,145

(or 4,144).

• If the final recommended sample size is not an integer, then the response can earn no more

than a P.

STATISTICS

2011 SCORING GUIDELINES (Form B)

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Question 5 (continued)

Each essentially correct (E) part counts as 1 point. Each partially correct (P) part counts as ½ point.

4 Complete Response

3 Substantial Response

2 Developing Response

1 Minimal Response

If a response is between two scores (fo

r example, 2½ points), use a holistic approach to decide whether to

score up or down, depending on the overall strength of the response and communication.

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STATISTICS

2011 SCORING COMMENTARY (Form B)

Question 5

Sample: 5A

Score: 4

Part (a) correctly identifies the procedure to be a “[o]ne sample z confidence interval for [a] proportion.” The

response notes that random sampling is given in the statement of the problem

and includes appropriate

checks of the sample-size conditions based on the observed counts of successes and failures. Step 1 of the

solution to part (a) was scored as essentially correct. In step 2 of part (a) the mechanics are shown and are

correct for the calculation of the interval identified in step 1, so step 2 of the solution to part (a) was scored as

essentially correct. After stating that the interval is a 99 percent confidence interval, the response notes that

“0.45 is not contained in the interval,” and so it is not reasonable to believe that 45 percent of the vaccine-

eligible people had received flu vaccine. The solution to step 3 of part (a) was scored as essentially correct.

The solution to part (b) incorporates 2.576, the correct critical value for a 99 percent confidence interval based

on a normal distribution, and also includes 0.5 as the value of the proportion. The conservative choice of 0.5

guarantees that the margin of error based on the resulting sample size calculation will be no greater than

0.02. Part (b) was scored as essentially correct. Because the three steps in part (a) were scored as essentially

correct and part (b) was also scored as essentially correct, the response earned a score of 4.

Sample: 5B

Score: 3

This response notes that “[t]he sample is random” but also that “2350 is more than 10% of the population.”

This sample-size restriction is both incomplete (because it does not restrict the numbers of successes and

failures) and incorrect (because it should read “less than” rather than “more than”). The procedure is

identified as a “one proportion z interval test.” Because of the incorrect sample-size condition, step 1 of

part (a) was scored as partially correct. Because the correct “one proportion z interval test” procedure is

named, and the correct interval is provided, step 2 of part (a) was scored as essentially correct. The response

then compares 0.45 to the resulting confidence interval by indicating that the interval of people that receive

this flu vaccine is less than 45 percent. The response includes context and a direct comparison of 0.45 to the

interval, so step 3 of part (a) was scored as essentially correct. The response states that “the belief is wrong,”

which is not necessarily correct, although the evidence indicates that 0.45 is not a plausible value for the true

proportion. In part (b) the response substitutes the observed proportion 0.41617 into the formula for the

margin of error. The observed proportion is not as conservative as using the value 0.5, so the resulting sample

size does not guarantee that the margin of error will be no greater than 0.02. Part (b) was scored partially

correct. Because one step in part (a) was scored as essentially correct, and part (b) was scored as essentially

correct, while two steps in part (a) were scored as partially correct, the response earned a score of 3.

Sample: 5C

Score: 2

The solution to part (a) includes no consideration of conditions for inference, so step 1 of part (a) was scored

as incorrect. The correct procedure (“1 proportion z interval”) is name

d, and the correct interval is calculated.

Step 2 of part (a) was thus scored as essentially correct. The response notes that 45 percent is not included in

the interval and that “we can’t say that 45% of vaccine-eligible people received flu vaccine.” This response

was scored essentially correct for step 3 of part (a). The solution in part (b) does not provide a formula,

includes 0.45 in the numerator of the standard error instead of the more statistically conservative value 0.5,

and does not include the square root in the standard error. Therefore, part (b) was scored as incorrect.

Because two steps in part (a) were scored as essentially correct, one step in part (a) was scored as incorrect,

and part (b) was scored as incorrect, the response earned a score of 2.

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