# 11.4: Test of Independence

Tests of independence involve using a contingency table of observed (data) values.

The test statistic for a test of independence is similar to that of a goodness-of-fit test:

$\sum_{(i \cdot j)} \frac{(O-E)^{2}}{E}$

where:

• $$O =$$ observed values
• $$E =$$ expected values
• $$i =$$ the number of rows in the table
• $$j =$$ the number of columns in the table

There are $$i \cdot j$$ terms of the form $$\frac{(O-E)^{2}}{E}$$.

The expected value for each cell needs to be at least five in order for you to use this test.

A test of independence determines whether two factors are independent or not. You first encountered the term independence in Probability Topics. As a review, consider the following example.

Example $$\PageIndex{1}$$

Suppose $$A =$$ a speeding violation in the last year and $$B =$$ a cell phone user while driving. If $$A$$ and $$B$$ are independent then $$P(A \text{ AND } B) = P(A)P(B)$$. $$A \text{ AND } B$$ is the event that a driver received a speeding violation last year and also used a cell phone while driving. Suppose, in a study of drivers who received speeding violations in the last year, and who used cell phone while driving, that 755 people were surveyed. Out of the 755, 70 had a speeding violation and 685 did not; 305 used cell phones while driving and 450 did not.

Let $$y =$$ expected number of drivers who used a cell phone while driving and received speeding violations.

If $$A$$ and $$B$$ are independent, then $$P(A \text{ AND } B) = P(A)P(B)$$. By substitution,

$\frac{y}{755} = \left(\frac{70}{755}\right)\left(\frac{305}{755}\right) \nonumber$

Solve for $$y$$:

$y = \frac{(70)(305)}{755} = 28.3 \nonumber$

About 28 people from the sample are expected to use cell phones while driving and to receive speeding violations.

In a test of independence, we state the null and alternative hypotheses in words. Since the contingency table consists of two factors, the null hypothesis states that the factors are independent and the alternative hypothesis states that they are not independent (dependent). If we do a test of independence using the example, then the null hypothesis is:

$$H_{0}$$: Being a cell phone user while driving and receiving a speeding violation are independent events.

If the null hypothesis were true, we would expect about 28 people to use cell phones while driving and to receive a speeding violation.

The test of independence is always right-tailed because of the calculation of the test statistic. If the expected and observed values are not close together, then the test statistic is very large and way out in the right tail of the chi-square curve, as it is in a goodness-of-fit.

The number of degrees of freedom for the test of independence is:

$df = (\text{number of columns} - 1)(\text{number of rows} - 1) \nonumber$

The following formula calculates the expected number ($$E$$):

$E = \frac{\text{(row total)(column total)}}{\text{total number surveyed}} \nonumber$

Exercise $$\PageIndex{1}$$

A sample of 300 students is taken. Of the students surveyed, 50 were music students, while 250 were not. Ninety-seven were on the honor roll, while 203 were not. If we assume being a music student and being on the honor roll are independent events, what is the expected number of music students who are also on the honor roll?

About 16 students are expected to be music students and on the honor roll.

Example $$\PageIndex{2}$$

In a volunteer group, adults 21 and older volunteer from one to nine hours each week to spend time with a disabled senior citizen. The program recruits among community college students, four-year college students, and nonstudents. In Table $$\PageIndex{1}$$ is a sample of the adult volunteers and the number of hours they volunteer per week.

Table $$\PageIndex{1}$$: Number of Hours Worked Per Week by Volunteer Type (Observed). The table contains observed (O) values (data).
Type of Volunteer 1–3 Hours 4–6 Hours 7–9 Hours Row Total
Community College Students 111 96 48 255
Four-Year College Students 96 133 61 290
Nonstudents 91 150 53 294
Column Total 298 379 162 839

Is the number of hours volunteered independent of the type of volunteer?

The observed table and the question at the end of the problem, "Is the number of hours volunteered independent of the type of volunteer?" tell you this is a test of independence. The two factors are number of hours volunteered and type of volunteer. This test is always right-tailed.

• $$H_{0}$$: The number of hours volunteered is independent of the type of volunteer.
• $$H_{a}$$: The number of hours volunteered is dependent on the type of volunteer.

The expected results are in Table $$\PageIndex{2}$$.

Table $$\PageIndex{2}$$: Number of Hours Worked Per Week by Volunteer Type (Expected). The table contains expected($$E$$) values (data).
Type of Volunteer 1-3 Hours 4-6 Hours 7-9 Hours
Community College Students 90.57 115.19 49.24
Four-Year College Students 103.00 131.00 56.00
Nonstudents 104.42 132.81 56.77

For example, the calculation for the expected frequency for the top left cell is

$E = \frac{(\text{row total})(\text{column total})}{\text{total number surveyed}} = \frac{(255)(298)}{839} = 90.57 \nonumber$

Calculate the test statistic: $$\chi^{2} = 12.99$$ (calculator or computer)

Distribution for the test: $$\chi^{2}_{4}$$

$df = (3 \text{ columns} – 1)(3 \text{ rows} – 1) = (2)(2) = 4 \nonumber$

Graph:

Probability statement: $$p\text{-value} = P(\chi^{2} > 12.99) = 0.0113$$

Compare $$\alpha$$ and the $$p\text{-value}$$: Since no $$\alpha$$ is given, assume $$\alpha = 0.05$$. $$p\text{-value} = 0.0113$$. $$\alpha > p\text{-value}$$.

Make a decision: Since $$\alpha > p\text{-value}$$, reject $$H_{0}$$. This means that the factors are not independent.

Conclusion: At a 5% level of significance, from the data, there is sufficient evidence to conclude that the number of hours volunteered and the type of volunteer are dependent on one another.

For the example in Table, if there had been another type of volunteer, teenagers, what would the degrees of freedom be?

USING THE TI-83, 83+, 84, 84+ CALCULATOR

Press the MATRX key and arrow over to EDIT. Press 1:[A]. Press 3 ENTER 3 ENTER. Enter the table values by row from Table. Press ENTER after each. Press 2nd QUIT. Press STAT and arrow over to TESTS. Arrow down to C:χ2-TEST. Press ENTER. You should see Observed:[A] and Expected:[B]. If necessary, use the arrow keys to move the cursor after Observed: and press 2nd MATRX. Press 1:[A] to select matrix A. It is not necessary to enter expected values. The matrix listed after Expected: can be blank. Arrow down to Calculate. Press ENTER. The test statistic is 12.9909 and the p-value = 0.0113. Do the procedure a second time, but arrow down to Draw instead of calculate.

Exercise $$\PageIndex{2}$$

The Bureau of Labor Statistics gathers data about employment in the United States. A sample is taken to calculate the number of U.S. citizens working in one of several industry sectors over time. Table $$\PageIndex{3}$$ shows the results:

Table $$\PageIndex{3}$$
Industry Sector 2000 2010 2020 Total
Nonagriculture wage and salary 13,243 13,044 15,018 41,305
Goods-producing, excluding agriculture 2,457 1,771 1,950 6,178
Services-providing 10,786 11,273 13,068 35,127
Agriculture, forestry, fishing, and hunting 240 214 201 655
Nonagriculture self-employed and unpaid family worker 931 894 972 2,797
Secondary wage and salary jobs in agriculture and private household industries 14 11 11 36
Secondary jobs as a self-employed or unpaid family worker 196 144 152 492
Total 27,867 27,351 31,372 86,590

We want to know if the change in the number of jobs is independent of the change in years. State the null and alternative hypotheses and the degrees of freedom.

• $$H_{0}$$: The number of jobs is independent of the year.
• $$H_{a}$$: The number of jobs is dependent on the year.
$$df = 12$$

Press the MATRX key and arrow over to EDIT. Press 1:[A]. Press 3 ENTER 3 ENTER. Enter the table values by row. Press ENTER after each. Press 2nd QUIT. Press STAT and arrow over to TESTS. Arrow down to c:$$\chi^{2}$$-TEST. Press ENTER. You should see Observed:[A] and Expected:[B]. Arrow down to Calculate. Press ENTER. The test statistic is 227.73 and the $$p\text{-value} = 5.90E - 42 = 0$$. Do the procedure a second time but arrow down to Draw instead of calculate.

Example $$\PageIndex{3}$$

De Anza College is interested in the relationship between anxiety level and the need to succeed in school. A random sample of 400 students took a test that measured anxiety level and need to succeed in school. Table shows the results. De Anza College wants to know if anxiety level and need to succeed in school are independent events.

Need to Succeed in School vs. Anxiety Level
Need to Succeed in School High
Anxiety
Med-high
Anxiety
Medium
Anxiety
Med-low
Anxiety
Low
Anxiety
Row Total
High Need 35 42 53 15 10 155
Medium Need 18 48 63 33 31 193
Low Need 4 5 11 15 17 52
Column Total 57 95 127 63 58 400
1. How many high anxiety level students are expected to have a high need to succeed in school?
2. If the two variables are independent, how many students do you expect to have a low need to succeed in school and a med-low level of anxiety?
3. $$E = \frac{(\text{row total})(\text{column total})}{\text{total surveyed}} =$$ ________
4. The expected number of students who have a med-low anxiety level and a low need to succeed in school is about ________.

Solution

a. The column total for a high anxiety level is 57. The row total for high need to succeed in school is 155. The sample size or total surveyed is 400.

$E = \frac{(\text{row total})(\text{column total})}{\text{total surveyed}} = \frac{155 \cdot 57}{400} = 22.09$

The expected number of students who have a high anxiety level and a high need to succeed in school is about 22.

b. The column total for a med-low anxiety level is 63. The row total for a low need to succeed in school is 52. The sample size or total surveyed is 400.

c. $$E = \frac{(\text{row total})(\text{column total})}{\text{total surveyed}} = 8.19$$

d. 8

Exercise $$\PageIndex{3}$$

Refer back to the information in Note. How many service providing jobs are there expected to be in 2020? How many nonagriculture wage and salary jobs are there expected to be in 2020?

12,727, 14,965

## References

1. DiCamilo, Mark, Mervin Field, “Most Californians See a Direct Linkage between Obesity and Sugary Sodas. Two in Three Voters Support Taxing Sugar-Sweetened Beverages If Proceeds are Tied to Improving School Nutrition and Physical Activity Programs.” The Field Poll, released Feb. 14, 2013. Available online at field.com/fieldpollonline/sub...rs/Rls2436.pdf (accessed May 24, 2013).
2. Harris Interactive, “Favorite Flavor of Ice Cream.” Available online at http://www.statisticbrain.com/favori...r-of-ice-cream (accessed May 24, 2013)
3. “Youngest Online Entrepreneurs List.” Available online at http://www.statisticbrain.com/younge...repreneur-list (accessed May 24, 2013).

## Review

To assess whether two factors are independent or not, you can apply the test of independence that uses the chi-square distribution. The null hypothesis for this test states that the two factors are independent. The test compares observed values to expected values. The test is right-tailed. Each observation or cell category must have an expected value of at least 5.

## Formula Review

Test of Independence

• The number of degrees of freedom is equal to $$(\text{number of columns - 1})(\text{number of rows - 1})$$.
• The test statistic is $$\sum_{(i \cdot j)} \frac{(O-E)^{2}}{E}$$ where $$O =$$ observed values, $$E =$$ expected values, $$i =$$ the number of rows in the table, and $$j =$$ the number of columns in the table.
• If the null hypothesis is true, the expected number $$E = \frac{(\text{row total})(\text{column total})}{\text{total surveyed}}$$.

Determine the appropriate test to be used in the next three exercises.

Exercise $$\PageIndex{4}$$

A pharmaceutical company is interested in the relationship between age and presentation of symptoms for a common viral infection. A random sample is taken of 500 people with the infection across different age groups.

a test of independence

Exercise $$\PageIndex{5}$$

The owner of a baseball team is interested in the relationship between player salaries and team winning percentage. He takes a random sample of 100 players from different organizations.

Exercise $$\PageIndex{6}$$

A marathon runner is interested in the relationship between the brand of shoes runners wear and their run times. She takes a random sample of 50 runners and records their run times as well as the brand of shoes they were wearing.

a test of independence

Use the following information to answer the next seven exercises: Transit Railroads is interested in the relationship between travel distance and the ticket class purchased. A random sample of 200 passengers is taken. Table $$\PageIndex{4}$$ shows the results. The railroad wants to know if a passenger’s choice in ticket class is independent of the distance they must travel.

Table $$\PageIndex{4}$$
Traveling Distance Third class Second class First class Total
1–100 miles 21 14 6 41
101–200 miles 18 16 8 42
201–300 miles 16 17 15 48
301–400 miles 12 14 21 47
401–500 miles 6 6 10 22
Total 73 67 60 200

Exercise $$\PageIndex{7}$$

State the hypotheses.

• $$H_{0}$$: _______
• $$H_{a}$$: _______

Exercise $$\PageIndex{8}$$

$$df =$$ _______

8

Exercise $$\PageIndex{9}$$

How many passengers are expected to travel between 201 and 300 miles and purchase second-class tickets?

Exercise $$\PageIndex{10}$$

How many passengers are expected to travel between 401 and 500 miles and purchase first-class tickets?

6.6

Exercise $$\PageIndex{11}$$

What is the test statistic?

Exercise $$\PageIndex{12}$$

What is the $$p\text{-value}$$?

0.0435

Exercise $$\PageIndex{13}$$

What can you conclude at the 5% level of significance?

Use the following information to answer the next eight exercises: An article in the New England Journal of Medicine, discussed a study on smokers in California and Hawaii. In one part of the report, the self-reported ethnicity and smoking levels per day were given. Of the people smoking at most ten cigarettes per day, there were 9,886 African Americans, 2,745 Native Hawaiians, 12,831 Latinos, 8,378 Japanese Americans and 7,650 whites. Of the people smoking 11 to 20 cigarettes per day, there were 6,514 African Americans, 3,062 Native Hawaiians, 4,932 Latinos, 10,680 Japanese Americans, and 9,877 whites. Of the people smoking 21 to 30 cigarettes per day, there were 1,671 African Americans, 1,419 Native Hawaiians, 1,406 Latinos, 4,715 Japanese Americans, and 6,062 whites. Of the people smoking at least 31 cigarettes per day, there were 759 African Americans, 788 Native Hawaiians, 800 Latinos, 2,305 Japanese Americans, and 3,970 whites.

Exercise $$\PageIndex{14}$$

Complete the table.

Table $$\PageIndex{5}$$: Smoking Levels by Ethnicity (Observed)
Smoking Level Per Day African American Native Hawaiian Latino Japanese Americans White TOTALS
1-10
11-20
21-30
31+
TOTALS

Table $$\PageIndex{5B}$$
Smoking Level Per Day African American Native Hawaiian Latino Japanese Americans White Totals
1-10 9,886 2,745 12,831 8,378 7,650 41,490
11-20 6,514 3,062 4,932 10,680 9,877 35,065
21-30 1,671 1,419 1,406 4,715 6,062 15,273
31+ 759 788 800 2,305 3,970 8,622
Totals 18,830 8,014 19,969 26,078 27,559 10,0450

Exercise $$\PageIndex{15}$$

State the hypotheses.

• $$H_{0}$$: _______
• $$H_{a}$$: _______

Exercise $$\PageIndex{16}$$

Enter expected values in Table. Round to two decimal places.

Calculate the following values:

Table $$\PageIndex{6}$$
Smoking Level Per Day African American Native Hawaiian Latino Japanese Americans White
1-10 7777.57 3310.11 8248.02 10771.29 11383.01
11-20 6573.16 2797.52 6970.76 9103.29 9620.27
21-30 2863.02 1218.49 3036.20 3965.05 4190.23
31+ 1616.25 687.87 1714.01 2238.37 2365.49

Exercise $$\PageIndex{17}$$

$$df =$$ _______

Exercise $$\PageIndex{18}$$

$$\chi^{2} \text{test statistic} =$$ ______

10,301.8

Exercise $$\PageIndex{19}$$

$$p\text{-value} =$$ ______

Exercise $$\PageIndex{20}$$

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

right

Exercise $$\PageIndex{21}$$

Graph the situation. Label and scale the horizontal axis. Mark the mean and test statistic. Shade in the region corresponding to the $$p\text{-value}$$.

State the decision and conclusion (in a complete sentence) for the following preconceived levels of $$\alpha$$.

Exercise $$\PageIndex{22}$$

$$\alpha = 0.05$$

1. Decision: ___________________
2. Reason for the decision: ___________________
3. Conclusion (write out in a complete sentence): ___________________

1. Reject the null hypothesis.
2. $$p\text{-value} < \alpha$$
3. There is sufficient evidence to conclude that smoking level is dependent on ethnic group.

Exercise $$\PageIndex{23}$$

$$\alpha = 0.05$$

1. Decision: ___________________
2. Reason for the decision: ___________________
3. Conclusion (write out in a complete sentence): ___________________

## Glossary

Contingency Table
a table that displays sample values for two different factors that may be dependent or contingent on one another; it facilitates determining conditional probabilities.