ch. 12 inference for proportions
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Ch. 12 Inference for Proportions. 12.1 Inference for a proportion Sample Proportion. Section 12.2. Comparing two proportions. Comparing Two Proportions. Comparisons between two percentages are much more common than questions about isolated percentages. And they are more interesting. - PowerPoint PPT PresentationTRANSCRIPT
Copyright © 2010 Pearson Education, Inc.
Ch. 12 Inference for Proportions
12.1 Inference for a proportion Sample Proportion
Slide 22 - 1
ˆsuccesses
pobservations
ˆ ˆ
ˆp q
SD pn
ˆ
ˆ( )
p
p pz
SD
ˆ
ˆp q
p zn
Copyright © 2010 Pearson Education, Inc.
Section 12.2
Comparing two proportions
Slide 22 - 2
Copyright © 2010 Pearson Education, Inc. Slide 22 - 3
Comparing Two Proportions
Comparisons between two percentages are much more common than questions about isolated percentages. And they are more interesting.
We often want to know how two groups differ, whether a treatment is better than a placebo control, or whether this year’s results are better than last year’s.
Copyright © 2010 Pearson Education, Inc. Slide 22 - 4
The Standard Deviation of the Difference Between Two Proportions
Proportions observed in independent random samples are independent. Thus, we can add their variances. So…
The standard deviation of the difference between two sample proportions is
Thus, the standard error is
1 1 2 21 2
1 2
ˆ ˆp q p q
SD p pn n
1 1 2 21 2
1 2
ˆ ˆ ˆ ˆˆ ˆ
p q p qSE p p
n n
Copyright © 2010 Pearson Education, Inc. Slide 22 - 5
Assumptions and Conditions
Independence Assumptions: Randomization Condition: The data in each
group should be drawn independently and at random from a homogeneous population or generated by a randomized comparative experiment.
The 10% Condition: If the data are sampled without replacement, the sample should not exceed 10% of the population.
Independent Groups Assumption: The two groups we’re comparing must be independent of each other.
Copyright © 2010 Pearson Education, Inc. Slide 22 - 6
Assumptions and Conditions (cont.)
Sample Size Condition: Each of the groups must be big enough… Success/Failure Condition: Both groups are big
enough that at least 10 successes and at least 10 failures have been observed in each.
Copyright © 2010 Pearson Education, Inc. Slide 22 - 7
Sample: Among randomly sampled teens aged 15-17, 57% of the 248 boys had posted online profiles, compared to 70% of the 256 girls. Can we use these results to make inferences about all 15-17 year-olds?
Copyright © 2010 Pearson Education, Inc. Slide 22 - 8
The Sampling Distribution
We already know that for large enough samples, each of our proportions has an approximately Normal sampling distribution.
The same is true of their difference.
Copyright © 2010 Pearson Education, Inc. Slide 22 - 9
The Sampling Distribution (cont.)
Provided that the sampled values are independent, the samples are independent, and the samples sizes are large enough, the sampling distribution of is modeled by a Normal model with Mean:
Standard deviation:
1 2p p
1 2ˆ ˆp p
1 1 2 21 2
1 2
ˆ ˆp q p q
SD p pn n
Copyright © 2010 Pearson Education, Inc. Slide 22 - 10
Two-Proportion z-Interval When the conditions are met, we are ready to
find the confidence interval for the difference of two proportions:
The confidence interval is
where
The critical value z* depends on the particular confidence level, C, that you specify.
1 1 2 21 2
1 2
ˆ ˆ ˆ ˆˆ ˆ
p q p qSE p p
n n
1 2 1 2ˆ ˆ ˆ ˆp p z SE p p
Copyright © 2010 Pearson Education, Inc. Slide 22 - 11
Sample: Among randomly sampled 15-17 year olds, 57% of the 248 boys had posted online profiles, compared to 70% of the 256 girls. We calculated the standard error for the difference in sample proportions to be .0425 and found that the assumptions and conditions required for inference were met. What does a confidence interval say about the difference in online behavior?
Copyright © 2010 Pearson Education, Inc. Slide 22 - 12
TI-Tips Find a Confidence Interval
STATS
TESTS
B:2-PropZInt
57% of the 248 boys = 141.36 (round to nearest int.)
70% of the 256 girls = 179.2 (round to nearest int.)
(Put data in, largest %, then smallest %)
Copyright © 2010 Pearson Education, Inc. Slide 22 - 13
Hypothesis Test for the Difference in Two Proportions
The typical hypothesis test for the difference in two proportions is the one of no difference. In symbols, H0: p1 – p2 = 0.
Since we are hypothesizing that there is no difference between the two proportions, that means that the standard deviations for each proportion are the same.
Since this is the case, we combine (pool) the counts to get one overall proportion.
Copyright © 2010 Pearson Education, Inc. Slide 22 - 19
TI-Tips Two-proportion z-test
Stat
Tests
6:2-PropZTest
Enter numbers for each group
Choose tail.
Determines the z-value and p-value!
Copyright © 2010 Pearson Education, Inc. Slide 22 - 20
What Can Go Wrong? Don’t use two-sample proportion methods when
the samples aren’t independent. These methods give wrong answers when the
independence assumption is violated. Don’t apply inference methods when there was
no randomization. Our data must come from representative
random samples or from a properly randomized experiment.