Confidence interval for the difference between proportions

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The approach that we used to solve this problem is valid when the following conditions are met.


Since the above requirements are satisfied, we can use the following four-step approach to construct a confidence interval.

1. Identify a sample statistic.

Since we are trying to estimate the difference between population proportions, we choose the difference between sample proportions as the sample statistic. Thus, the sample statistic is \(p_{boy} - p_{girl} = 0.4 - 0.3 = 0.1\).

2. Select a confidence level.

In this analysis, the confidence level is defined for us in the problem. We are working with a 90% confidence level.

3. Find the margin of error.

Find standard deviation or standard error.

Since we do not know the population proportions, we cannot compute the standard deviation; instead, we compute the standard error. And since each population is more than 20 times larger than its sample, we can use the following formula to compute the standard error (SE) of the difference between proportions:

\[ SE= \sqrt{\frac{p_1(1-p_1)}{n_1}+\frac{p_2(1-p_2)}{n_2}}\] where \(p_1\) is the sample proportion for sample 1, \(n_1\) is the sample size from population 1, \(p_2\) is the sample proportion for sample 2 and \(n_2\) is the sample size from population 2.

\[ SE= \sqrt{\frac{0.4(1-0.4)}{400}+\frac{0.3(1-0.3)}{300}}=0.036\]

Find the critical probability(p*):


The critical value is the z statistic having a cumulative probability equal to 0.95.

We can get the critical value using the following R code.


Alternatively, we find that the critical value is 1.645 from the normal Distribution table.

The graph shows the \(\alpha\) values are the tail areas of the distribution.

Compute margin of error(ME):

\[ME=critical\ value \times SE\] \[ME=1.645 \times 0.036=0.059\]

Specify the confidence interval. The range of the confidence interval is defined by the sample statistic \(\pm\)margin of error. And the uncertainty is denoted by the confidence level.

4. Confidence interval of the proportion

Therefore, the 90% confidence interval is 0.04 to 0.16. That is, we are 90% confident that the true proportion is in the range 0.04 to 0.16. Since both ends of the confidence interval are positive, we can conclude that more boys than girls choose Superman as their favorite cartoon character.

Result of propCI()

# A tibble: 1 × 2
  x     y    
  <lgl> <lgl>
1 NA    NA   

  alpha  p1  p2  n1  n2  DF  pd         se critical         ME      lower
1   0.1 0.4 0.3 400 300 698 0.1 0.03605551 1.644854 0.05930604 0.04069396
     upper                     CI   ppooled   sepooled       z      pvalue
1 0.159306 0.10 [90CI 0.04; 0.16] 0.3571429 0.03659625 2.73252 0.006285182
1   two.sided

propCI(n1 = 400, n2 = 300, p1 = 0.4, p2 = 0.3, alpha = 0.1)

[1] "propdiff"


The contents of this document are modified from Berman H.B., “AP Statistics Tutorial”, [online] Available at: URL[Accessed Data: 1/23/2022].