11.6 Test of a single variance

 Page 1 / 22

A test of a single variance assumes that the underlying distribution is normal . The null and alternative hypotheses are stated in terms of the population variance (or population standard deviation). The test statistic is:

$\frac{\left(n-1\right){s}^{2}}{{\sigma }^{2}}$

where:

• n = the total number of data
• s 2 = sample variance
• σ 2 = population variance

You may think of s as the random variable in this test. The number of degrees of freedom is df = n - 1. A test of a single variance may be right-tailed, left-tailed, or two-tailed. [link] will show you how to set up the null and alternative hypotheses. The null and alternative hypotheses contain statements about the population variance.

Math instructors are not only interested in how their students do on exams, on average, but how the exam scores vary. To many instructors, the variance (or standard deviation) may be more important than the average.

Suppose a math instructor believes that the standard deviation for his final exam is five points. One of his best students thinks otherwise. The student claims that the standard deviation is more than five points. If the student were to conduct a hypothesis test, what would the null and alternative hypotheses be?

Even though we are given the population standard deviation, we can set up the test using the population variance as follows.

• H 0 : σ 2 = 5 2
• H a : σ 2 >5 2

Try it

A SCUBA instructor wants to record the collective depths each of his students dives during their checkout. He is interested in how the depths vary, even though everyone should have been at the same depth. He believes the standard deviation is three feet. His assistant thinks the standard deviation is less than three feet. If the instructor were to conduct a test, what would the null and alternative hypotheses be?

H 0 : σ 2 = 3 2

H a : σ 2 <3 2

With individual lines at its various windows, a post office finds that the standard deviation for normally distributed waiting times for customers on Friday afternoon is 7.2 minutes. The post office experiments with a single, main waiting line and finds that for a random sample of 25 customers, the waiting times for customers have a standard deviation of 3.5 minutes.

With a significance level of 5%, test the claim that a single line causes lower variation among waiting times (shorter waiting times) for customers .

Since the claim is that a single line causes less variation, this is a test of a single variance. The parameter is the population variance, σ 2 , or the population standard deviation, σ .

Random Variable: The sample standard deviation, s , is the random variable. Let s = standard deviation for the waiting times.

• H 0 : σ 2 = 7.2 2
• H a : σ 2 <7.2 2

The word "less" tells you this is a left-tailed test.

Distribution for the test: ${\chi }_{24}^{2}$ , where:

• n = the number of customers sampled
• df = n – 1 = 25 – 1 = 24

Calculate the test statistic:

where n = 25, s = 3.5, and σ = 7.2.

Graph:

Probability statement: p -value = P ( χ 2 <5.67) = 0.000042

Compare α and the p -value:

• α = 0.05
• p -value = 0.000042
• α > p -value

Make a decision: Since α > p -value, reject H 0 . This means that you reject σ 2 = 7.2 2 . In other words, you do not think the variation in waiting times is 7.2 minutes; you think the variation in waiting times is less.

Conclusion: At a 5% level of significance, from the data, there is sufficient evidence to conclude that a single line causes a lower variation among the waiting times or with a single line, the customer waiting times vary less than 7.2 minutes.

In 2nd DISTR , use 7:χ2cdf . The syntax is (lower, upper, df) for the parameter list. For [link] , χ2cdf(-1E99,5.67,24) . The p -value = 0.000042.

Try it

The FCC conducts broadband speed tests to measure how much data per second passes between a consumer’s computer and the internet. As of August of 2012, the standard deviation of Internet speeds across Internet Service Providers (ISPs) was 12.2 percent. Suppose a sample of 15 ISPs is taken, and the standard deviation is 13.2. An analyst claims that the standard deviation of speeds is more than what was reported. State the null and alternative hypotheses, compute the degrees of freedom, the test statistic, sketch the graph of the p -value, and draw a conclusion. Test at the 1% significance level.

H 0 : σ 2 = 12.2 2

H a : σ 2 >12.2 2
df = 14
chi 2 test statistic = 16.39

The p -value is 0.2902, so we decline to reject the null hypothesis. There is not enough evidence to suggest that the variance is greater than 12.2 2 .

In 2nd DISTR , use7: χ2cdf . The syntax is (lower, upper, df) for the parameter list. χ2cdf(16.39,10^99,14) . The p -value = 0.2902.

References

“AppleInsider Price Guides.” Apple Insider, 2013. Available online at http://appleinsider.com/mac_price_guide (accessed May 14, 2013).

Data from the World Bank, June 5, 2012.

Chapter review

To test variability, use the chi-square test of a single variance. The test may be left-, right-, or two-tailed, and its hypotheses are always expressed in terms of the variance (or standard deviation).

Formula review

${\chi }^{2}=$ $\frac{\left(n-1\right)\cdot {s}^{2}}{{\sigma }^{2}}$ Test of a single variance statistic where:
n : sample size
s : sample standard deviation
σ : population standard deviation

df = n – 1 Degrees of freedom

Test of a single variance

• Use the test to determine variation.
• The degrees of freedom is the number of samples – 1.
• The test statistic is $\frac{\left(n–1\right)\cdot {s}^{2}}{{\sigma }^{2}}$ , where n = the total number of data, s 2 = sample variance, and σ 2 = population variance.
• The test may be left-, right-, or two-tailed.

Use the following information to answer the next three exercises: An archer’s standard deviation for his hits is six (data is measured in distance from the center of the target). An observer claims the standard deviation is less.

What type of test should be used?

a test of a single variance

State the null and alternative hypotheses.

Is this a right-tailed, left-tailed, or two-tailed test?

a left-tailed test

Use the following information to answer the next three exercises: The standard deviation of heights for students in a school is 0.81. A random sample of 50 students is taken, and the standard deviation of heights of the sample is 0.96. A researcher in charge of the study believes the standard deviation of heights for the school is greater than 0.81.

What type of test should be used?

State the null and alternative hypotheses.

H 0 : σ 2 = 0.81 2 ;

H a : σ 2 >0.81 2

df = ________

Use the following information to answer the next four exercises: The average waiting time in a doctor’s office varies. The standard deviation of waiting times in a doctor’s office is 3.4 minutes. A random sample of 30 patients in the doctor’s office has a standard deviation of waiting times of 4.1 minutes. One doctor believes the variance of waiting times is greater than originally thought.

What type of test should be used?

a test of a single variance

What is the test statistic?

What is the p -value?

0.0542

What can you conclude at the 5% significance level?

what is statistics
Hey
Shahidul
hi
Alex
Waz up dude
Shahidul
not much
Alex
Just woke up
Alex
hi
Amarachukwu
how can I get a use of statistics of the following areas physical science biological sciences industry
let make a research I will inform u
Nanfuna
what is the relation of state with chemistry
relation with chemistry
Anila
how to solve difficult questions?
Excel
The thread life of a particular brand of tyre is a random variable best described by a normal distribution with a mean of 60,000 miles and a standard deviation of 8,300 miles. If the manufacturer warrants the tyre for the first 45,000 miles, what proportion of the tire will need to be replaced under
am tired of notes staff alone.Edify the book mathematically
A production company produces 500 COVID_19 sanitizers and find out that 25% of the sanitizers are defectives if 4 sanitizers are selected at random 1-compute the probability of having 0, 1, 2, 3, 4 2-compute the probability of having _ A_between 1 and 4 B_between 2 and 4 inclusive
between 1and4
Alhassan
1 - (0 - 31,641%), (1 - 42,188%), (2 - 21,094%), (3 - 4,688%), (4 - 0,391%) 2 - (1-4 -> 25,781%) (2-|4 -> 26,1172%)
Luis
what is the basis of z table?
The manager of pharmaceutical company here in kaduna assumes the company's employees are honest. However, there have been many shortage from the cash register lately. There is only one employee who could have taken money from the register during this period. Realizing that the shortage might have resulted from the employee inadvertently giving incorrect change to customers, the employee does not know whether to forget the situation or accuse the employee of theft.
In words, what are the null and alternative hypothesis? Explain your choices. a) What constitutes a Type I error in this problem? b) What is a Type II error? Which do you think is more serious? Explain
A box contain three items that are labelled A,B,C.2 items are selected at random (without selecting) from the boxes, list all possible outcome for these experiment, write the sample space
the disadvantage of using stem and leaf graph over others?
frequency is not known
Dexter
yea
Nanfuna
what is data in shot
what is data set
A recent study indicated that 30% of the 100 women over age 55 in the study were widows. a. How large a sample must one take to be 95% confident that the estimate is within 0.05 of the true proportion of women over 55 who are widows? [4] b. If no estimate of the sample proportion is available,
333
Dexter
what is critical Value
critical value is a point on the test distribution that is compared to the test statistic to determine whether to reject the null hypothesis.
New
A college register has received numerous complaints about the online registration procedure at her college, alleging that the system is slow, confusing, and error prone. She wants to estimate the proportion of all students at this college who are dissatisfied with the online registration proc
Linda
A recent study indicated that 30% of the 100 women over age 55 in the study were widows. a. How large a sample must one take to be 95% confident that the estimate is within 0.05 of the true proportion of women over 55 who are widows? [4] b. If no estimate of the sample proportion is Available ho
Linda