QCAA Mathematical Methods Interval estimates for proportions
15 sample questions with marking guides and sample answers
Assuming the approximate normality of sample proportions ( and ) and based on two independent samples, the approximate confidence interval for the difference of two proportions is given by
If the approximate confidence interval for the difference between two proportions does not contain 0, this provides evidence that the two proportions are not equal.
The data in the table shows the observed frequencies of two drink preferences for independent samples of people who live in Town A and Town B.
| Town | Tea | Coffee | Total |
|---|---|---|---|
| A | 111 | 105 | 216 |
| B | 150 | 107 | 257 |
Using the approximate 99% confidence interval for the difference of two proportions, determine if there is evidence to conclude that drink preference is associated with the town where the person lives.
Reveal Answer
proportion of Town A who prefer to drink tea
proportion of Town B who prefer to drink tea
The sample proportions are:
Using the 99% confidence interval for the difference of two proportions
This interval contains zero; therefore, there is no evidence in the data to say that the two proportions are different, i.e. preference to drink tea does not depend on where the person lives.
| Descriptor | Marks |
|---|---|
correctly determines the sample proportions | 1 |
establishes confidence interval for the difference of two proportions | 1 |
determines 99% confidence interval | 1 |
interprets 99% confidence interval to determine equality of proportions | 1 |
shows logical organisation communicating key steps | 1 |
Let be the random variable that represents the sample proportion of households in a given suburb that have solar panels installed.
From a sample of randomly selected households in a given suburb, an approximate 95% confidence interval for the proportion of households having solar panels installed was determined to be .
Use to approximate the 95% confidence interval.
Find the value of that was used to obtain this approximate 95% confidence interval.
Reveal Answer
| Descriptor | Marks |
|---|---|
Calculates the correct value of | 1 |
Find the size of the sample from which this 95% confidence interval was obtained.
Reveal Answer
| Descriptor | Marks |
|---|---|
Sets up a correct equation involving using the margin of error or confidence interval bounds | 1 |
Calculates the correct sample size, | 1 |
A larger sample of households is selected, with a sample size four times the original sample.
The sample proportion of households having solar panels installed is found to be the same.
By what factor will the increased sample size affect the width of the confidence interval?
Reveal Answer
Confidence interval width is halved (reduced or decreased by a factor of 2; altered by a factor of ).
| Descriptor | Marks |
|---|---|
States the correct factor by which the width is affected (e.g., , halved, or decreased by a factor of 2) | 1 |
A survey was conducted to understand whether people support a new policy.
Using a z-score of 2, the approximate confidence interval for the population proportion of people who support the policy was calculated as .
Determine the margin of error.
Reveal Answer
The confidence interval corresponds to , where E is the margin of error about .
subtracting:
| Descriptor | Marks |
|---|---|
Correctly determines the margin of error | 1 |
Determine the number of people surveyed.
Reveal Answer
Upper CI value =
25 people were surveyed.
| Descriptor | Marks |
|---|---|
Determines the value of the sample proportion | 1 |
Substitutes and z-score into the confidence interval formula | 1 |
Determines the number of people surveyed | 1 |
An organisation randomly surveyed 1000 Australian adults and found that 55% of those surveyed were happy with their level of physical activity.
An approximate 95% confidence interval for the percentage of Australian adults who were happy with their level of physical activity is closest to
Reveal Answer
This interval is incorrect because it does not center around the sample proportion of 55%.
This interval is too wide. It corresponds to an approximate 99% confidence interval, which uses a z-score of 2.576 instead of 1.96.
This interval is too narrow. It corresponds to an approximate 90% confidence interval, which uses a z-score of 1.645 instead of 1.96.
This is correct. The 95% confidence interval is calculated using , which gives , or .
This interval is much too wide. It represents the 95% confidence interval if the sample size was instead of .
A box contains many coloured glass beads.
A random sample of 48 beads is selected and it is found that the proportion of blue-coloured beads in this sample is 0.125
Based on this sample, a 95% confidence interval for the proportion of blue-coloured glass beads is
(0.0314, 0.2186)
(0.0465, 0.2035)
(0.0018, 0.2482)
(0.0896, 0.1604)
(0.0264, 0.2136)
Reveal Answer
(0.0314, 0.2186)
This is correct. The 95% confidence interval is calculated using the formula , where , , and the critical value .
(0.0465, 0.2035)
This is incorrect. This interval uses a z-score of approximately 1.645, which corresponds to a 90% confidence interval rather than the requested 95%.
(0.0018, 0.2482)
This is incorrect. This interval uses a z-score of approximately 2.58, which calculates a 99% confidence interval instead of a 95% confidence interval.
(0.0896, 0.1604)
This is incorrect. This interval is much too narrow, likely resulting from an error in calculating the standard error or omitting the z-score multiplier.
(0.0264, 0.2136)
This is incorrect. This interval does not match the correct calculation for a 95% confidence interval and likely stems from an arithmetic error.
A sample of size was used to estimate a population proportion. An approximate margin of error of 3% was calculated using . Given the sample proportion was 0.6, determine .
Reveal Answer
This option incorrectly inverts the formula. The sample size is calculated by dividing the variance estimate by the squared term , not the other way around.
Rearranging the margin of error formula to solve for gives . With , the numerator is .
This option incorrectly inverts the fraction and uses the wrong value for the product . The calculation equals , not .
While the structure of the formula is correct, the numerator is calculated incorrectly. The value of is , not .
A firm aims to have 95% confidence in estimating the proportion of office workers who respond to an email in less than an hour to within .
A survey has never been undertaken before, so no past data is available.
The firm believes that if the proportion is 0.5, then this will result in the largest variability in the sample proportion.
Based on this, determine the sample size needed using the approximate value of for the 95% confidence interval.
Justify the choice of 0.5 for the proportion.
Reveal Answer
interval margin
rearranging
The largest sample size will result when is maximised in the numerator, therefore generating largest value.
Maximum occurs at
| Descriptor | Marks |
|---|---|
correctly selects the interval margin formula | 1 |
substitutes values into the formula | 1 |
determines sample size n | 1 |
verifies the firm's decision to use using mathematical reasoning | 1 |
A streaming service surveyed a random sample of 100 customers, and 60 customers said that they prefer to watch movies rather than miniseries.
Based on these results, the 95% confidence interval for the proportion of customers who prefer to watch movies was (0.5, 0.7).
Which statement shows the correct interpretation of this outcome?
The streaming service can be 95% confident that the proportion of customers who prefer to watch movies is between 50% and 70%.
There is a probability between 50% and 70% that most customers prefer to watch movies 95% of the time.
60% of the sampled customers prefer to watch movies between 50% and 70% of the time.
60 customers in the random sample prefer to watch movies 95% of the time.
Reveal Answer
The streaming service can be 95% confident that the proportion of customers who prefer to watch movies is between 50% and 70%.
This is the correct interpretation. A 95% confidence interval means we can be 95% confident that the true population proportion lies within the calculated interval, which is 50% to 70%.
There is a probability between 50% and 70% that most customers prefer to watch movies 95% of the time.
This is incorrect because a confidence interval estimates the true population proportion, not the probability or frequency of how often customers prefer movies.
60% of the sampled customers prefer to watch movies between 50% and 70% of the time.
This is incorrect. The interval (0.5, 0.7) represents the estimated range for the true population proportion, not the percentage of time individuals prefer to watch movies.
60 customers in the random sample prefer to watch movies 95% of the time.
This is incorrect because the 95% refers to the confidence level of the interval estimating the population proportion, not the percentage of time customers prefer movies.
A teacher conducted a survey about student involvement in school activity groups. They found that of a random sample of 25 students, five students were members of the environment group.
Determine the approximate confidence interval for the proportion of students at the school who are members of the environment group, using . Express your answer using fractions in simplified form.
Reveal Answer
Similarly the upper bound is
Confidence interval
| Descriptor | Marks |
|---|---|
correctly determines the sample proportion | 1 |
substitutes the sample size, sample proportion and z-value into at least one end of approximate confidence interval formula | 1 |
determines the approximate margin of error | 1 |
determines the lower and upper end of the approximate confidence interval | 1 |
In a survey, 80 respondents exercised daily, while 120 did not. When calculating the approximate 95% confidence interval for the proportion of people who exercise daily, the margin of error is
Reveal Answer
This is the correct formula for the margin of error. The sample proportion is , the total sample size is , and the critical value () for a 95% confidence interval is approximately 1.96.
This option incorrectly uses the confidence level (0.95) as the multiplier instead of the critical -score. For a 95% confidence interval, the critical value is 1.96.
This option uses an incorrect sample size and proportion. The denominator must be the total number of respondents (), not just those who did not exercise (), and the proportion is calculated relative to that total.
This option uses the wrong multiplier (0.95 instead of 1.96) and incorrect values for the sample size and proportion. The sample size should be the total number of respondents ().
The results of an employee satisfaction survey of 500 employees at a large company are presented to board members.
The results include a 95% confidence interval for the proportion of satisfied employees. The lower end of the confidence interval is 0.648.
A board member would like to use the survey results to make the claim that the proportion of the satisfied employees in the entire company is larger than 75%.
Evaluate the reasonableness of the claim.
Reveal Answer
Set up the 95% confidence interval:
Use a GDC with a 95% central area, and
The lower cut-off of the 75% CI is given by:
Solving for using a GDC:
Substituting into the upper CI formula:
The 95% CI is therefore:
The survey suggests we can be 95% confident the population proportion is between 0.648 and 0.729, i.e. between 64.8% and 72.9%.
The claim of 75% for the population proportion is outside of the 95% confidence interval in the survey, so the claim is not reasonable.
| Descriptor | Marks |
|---|---|
correctly determines the z-score for a 95% confidence interval | 1 |
determines an equation involving the lower cut-off of the CI formula | 1 |
determines the sample proportion for the survey results | 1 |
determines the upper end of the 95% confidence interval | 1 |
determines if the claim is reasonable | 1 |
A random sample of people were surveyed about the most important factor when deciding where to shop. The results appear in the table.
| Factor | Percentage (%) |
|---|---|
| Price | 40 |
| Quality of merchandise | 30 |
| Service | 15 |
| Shopping environment | 15 |
If the sample size was 1200, the approximate 95% confidence interval for the proportion of people who identified price as the most important factor is
(0.395, 0.405)
(0.386, 0.414)
(0.377, 0.423)
(0.372, 0.428)
Reveal Answer
(0.395, 0.405)
This interval is too narrow and does not account for the standard error associated with the sample size. It implies a margin of error of only , which is mathematically incorrect for these parameters.
(0.386, 0.414)
This interval represents approximately a 68% confidence level. It uses a margin of error of 1 standard error () instead of the required for 95% confidence.
(0.377, 0.423)
This interval corresponds to a 90% confidence level. It uses a critical value of instead of the required for a 95% confidence interval.
(0.372, 0.428)
Using the formula with and , the margin of error is , resulting in the interval .
Two types of material (A and B) are being tested for their ability to withstand different temperatures. A random selection of both materials was subjected to extreme temperature changes and then classified according to their condition after they were removed from the testing facility. The results are shown in the table.
| Material | ||
|---|---|---|
| A | B | |
| Broke completely | 25 | 43 |
| Showed defects | 35 | 38 |
| Remained intact | 35 | 24 |
| Total | 95 | 105 |
An approximate 95% confidence interval for the probability that material A will break completely or show defects is given by
The values of and are
and 95
and 95
and 95
and 200
Reveal Answer
and 95
This is the correct answer. The sample size is the total number of Material A items tested (). The proportion is the sum of Material A items that broke completely () or showed defects () divided by the sample size, resulting in .
and 95
This option incorrectly calculates the proportion by using the grand total of all materials () in the denominator. Since the question asks specifically about the probability for Material A, the denominator must be the total count of Material A ().
and 95
This option uses incorrect values for the proportion. It appears to use the total sample size () in the denominator and likely aggregates defects across both materials, rather than restricting the calculation to the specific outcomes and sample size of Material A.
and 200
This option incorrectly identifies the sample size as the total for both materials (). Since the inference is about Material A specifically, should be the total count of Material A ().
In a survey of 326 lecturers, 303 said that on at least one occasion a mobile phone had rung in a lecture they were giving.
Determine the sample size required to conduct a follow-up survey that provides 95% confidence that this one-point estimate is correct to within of the population proportion.
Reveal Answer
Using confidence interval formula (margin of error)
Using GDC
a sample of 630 lecturers would be required.
| Descriptor | Marks |
|---|---|
Correctly determines the sample proportion | 1 |
Establishes equation in | 1 |
Determines | 1 |
Determines reasonable value of | 1 |
A random sample of Victorian households is taken to estimate the proportion of all Victorian households that have vegetable gardens. The approximate 95% confidence interval calculated using this sample is , correct to three decimal places.
The number of households, , in the sample is
Reveal Answer
Incorrect. Substituting into the margin of error formula yields approximately , which is much larger than the required margin of error of .
Incorrect. Substituting into the margin of error formula yields approximately , which does not match the interval's actual margin of error of .
Correct. The sample proportion is the midpoint and the margin of error is . Solving the margin of error formula gives .
Incorrect. Substituting into the margin of error formula yields approximately , which is smaller than the required margin of error of .