Statistics

# 10.1Two Population Means with Unknown Standard Deviations

Statistics10.1 Two Population Means with Unknown Standard Deviations

1. The two independent samples are simple random samples from two distinct populations.
2. For the two distinct populations
• if the sample sizes are small, the distributions are important (should be normal), and
• if the sample sizes are large, the distributions are not important (need not be normal)
The test comparing two independent population means with unknown and possibly unequal population standard deviations is called the Aspin-Welch t-test. The degrees of freedom formula was developed by Aspin-Welch.

The comparison of two population means is very common. A difference between the two samples depends on both the means and the standard deviations. Very different means can occur by chance if there is great variation among the individual samples. To account for the variation, we take the difference of the sample means, $X ¯ 1 − X ¯ 2 X ¯ 1 − X ¯ 2$, and divide by the standard error to standardize the difference. The result is a t-score test statistic.

Because we do not know the population standard deviations, we estimate them using the two sample standard deviations from our independent samples. For the hypothesis test, we calculate the estimated standard deviation, or standard error, of the difference in sample means, $X ¯ 1 − X ¯ 2 . X ¯ 1 − X ¯ 2 .$

The standard error is calculated as follows:

$( s 1 ) 2 n 1 + ( s 2 ) 2 n 2 ( s 1 ) 2 n 1 + ( s 2 ) 2 n 2$
10.1

The test statistic (t-score) is calculated as follows:

$( x ¯ 1 – x ¯ 2 )–( μ 1 – μ 2 ) ( s 1 ) 2 n 1 + ( s 2 ) 2 n 2 ( x ¯ 1 – x ¯ 2 )–( μ 1 – μ 2 ) ( s 1 ) 2 n 1 + ( s 2 ) 2 n 2$
where
• s1 and s2, the sample standard deviations, are estimates of σ1 and σ2, respectively,
• σ1 and σ1 are the unknown population standard deviations,
• $x ¯ 1 x ¯ 1$ and $x ¯ 2 x ¯ 2$ are the sample means, and
• μ1 and μ2 are the population means.

The number of degrees of freedom (df) requires a somewhat complicated calculation. However, a computer or calculator calculates it easily. The df are not always a whole number. The test statistic calculated previously is approximated by the Student’s t-distribution with df as follows:

Degrees of freedom

$df= ( ( s 1 ) 2 n 1 + ( s 2 ) 2 n 2 ) 2 ( 1 n 1 –1 ) ( ( s 1 ) 2 n 1 ) 2 +( 1 n 2 –1 ) ( ( s 2 ) 2 n 2 ) 2 df= ( ( s 1 ) 2 n 1 + ( s 2 ) 2 n 2 ) 2 ( 1 n 1 –1 ) ( ( s 1 ) 2 n 1 ) 2 +( 1 n 2 –1 ) ( ( s 2 ) 2 n 2 ) 2$

When both sample sizes n1 and n2 are five or larger, the Student’s t approximation is very good. Notice that the sample variances (s1)2 and (s2)2 are not pooled. (If the question comes up, do not pool the variances.)

It is not necessary to compute this by hand. A calculator or computer easily computes it.

## Example 10.1

### Independent groups

The average amount of time boys and girls aged 7 to 11 spend playing sports each day is believed to be the same. A study is done and data are collected, resulting in the data in Table 10.1. Each populations has a normal distribution.

Sample Size Average Number of Hours Playing Sports per Day Sample Standard Deviation
Girls 9 2 0.866
Boys 16 3.2 1.00
Table 10.1

### Problem

Is there a difference in the mean amount of time boys and girls aged 7 to 11 play sports each day? Test at the 5 percent level of significance.

## Try It 10.1

Two samples are shown in Table 10.2. Both have normal distributions. The means for the two populations are thought to be the same. Is there a difference in the means? Test at the 5 percent level of significance.

Sample Size Sample Mean Sample Standard Deviation
Population A 25 5 1
Population B 16 4.7 1.2
Table 10.2

## NOTE

When the sum of the sample sizes is larger than 30 (n1 + n2 > 30), you can use the normal distribution to approximate the Student’s t.

## Example 10.2

A study is done by a community group in two neighboring colleges to determine which one graduates students with more math classes. College A samples 11 graduates. Their average is 4 math classes with a standard deviation of 1.5 math classes. College B samples nine graduates. Their average is 3.5 math classes with a standard deviation of 1 math class. The community group believes that a student who graduates from College A has taken more math classes, on average. Both populations have a normal distribution. Test at a 1 percent significance level. Answer the following questions:

### Problem

a. Is this a test of two means or two proportions?

### Problem

b. Are the populations standard deviations known or unknown?

### Problem

c. Which distribution do you use to perform the test?

### Problem

d. What is the random variable?

### Problem

e. What are the null and alternate hypotheses? Write the null and alternate hypotheses in symbols.

### Problem

f. Is this test right-, left-, or two-tailed?

### Problem

g. What is the p-value?

### Problem

h. Do you reject or not reject the null hypothesis?

i. Conclusion:

## Try It 10.2

A study is done to determine if Company A retains its workers longer than Company B. Company A samples 15 workers, and their average time with the company is 5 years with a standard deviation of 1.2. Company B samples 20 workers, and their average time with the company is 4.5 years with a standard deviation of 0.8. The populations are normally distributed.

1. Are the population standard deviations known?
2. Conduct an appropriate hypothesis test. At the 5 percent significance level, what is your conclusion?

## Example 10.3

A professor at a large community college wanted to determine whether there is a difference in the means of final exam scores between students who took his statistics course online and the students who took his face-to-face statistics class. He believed that the mean of the final exam scores for the online class would be lower than that of the face-to-face class. Was the professor correct? The randomly selected 30 final exam scores from each group are listed in Table 10.3 and Table 10.4.

 67.6 41.2 85.3 55.9 82.4 91.2 73.5 94.1 64.7 64.7 70.6 38.2 61.8 88.2 70.6 58.8 91.2 73.5 82.4 35.5 94.1 88.2 64.7 55.9 88.2 97.1 85.3 61.8 79.4 79.4
Table 10.3 Online Class
 77.9 95.3 81.2 74.1 98.8 88.2 85.9 92.9 87.1 88.2 69.4 57.6 69.4 67.1 97.6 85.9 88.2 91.8 78.8 71.8 98.8 61.2 92.9 90.6 97.6 100 95.3 83.5 92.9 89.4
Table 10.4 Face-to-Face Class

### Problem

Is the mean of the final exam scores of the online class lower than the mean of the final exam scores of the face-to-face class? Test at a 5 percent significance level. Answer the following questions:

1. Is this a test of two means or two proportions?
2. Are the population standard deviations known or unknown?
3. Which distribution do you use to perform the test?
4. What is the random variable?
5. What are the null and alternative hypotheses? Write the null and alternative hypotheses in words and in symbols.
6. Is this test right-, left-, or two-tailed?
7. What is the p-value?
8. Do you reject or not reject the null hypothesis?
9. At the _____ level of significance, from the sample data, there ______ (is/is not) sufficient evidence to conclude that ______.

(See the conclusion in Example 10.2, and write yours in a similar fashion.)

## Using the TI-83, 83+, 84, 84+ Calculator

First put the data for each group into two lists (such as L1 and L2). Press STAT. Arrow over to TESTS and press 4:2SampTTest. Make sure Data is highlighted and press ENTER. Arrow down and enter L1 for the first list and L2 for the second list. Arrow down to μ1: and arrow to < μ2 (less than). Press ENTER. Arrow down to Pooled: No. Press ENTER. Arrow down to Calculate and press ENTER.

## Note

Be careful not to mix up the information for Group 1 and Group 2!

Cohen’s Standards for Small, Medium, and Large Effect SizesCohen’s d is a measure of effect size based on the differences between two means. Cohen’s d, named for U.S. statistician Jacob Cohen, measures the relative strength of the differences between the means of two populations based on sample data. The calculated value of effect size is then compared to Cohen’s standards of small, medium, and large effect sizes.

Size of Effect d
Small 0.2
medium 0.5
Large 0.8
Table 10.5 Cohen’s Standard Effect Sizes

Cohen’s d is the measure of the difference between two means divided by the pooled standard deviation: $d= x ¯ 1 – x ¯ 2 s pooled d= x ¯ 1 – x ¯ 2 s pooled$ where $s pooled = ( n 1 –1) s 1 2 +( n 2 –1) s 2 2 n 1 + n 2 –2 s pooled = ( n 1 –1) s 1 2 +( n 2 –1) s 2 2 n 1 + n 2 –2$.

## Example 10.4

### Problem

Calculate Cohen’s d for Example 10.2. Is the size of the effect small, medium, or large? Explain what the size of the effect means for this problem.

## Example 10.5

### Problem

Calculate Cohen’s d for Example 10.3. Is the size of the effect small, medium, or large? Explain what the size of the effect means for this problem.

## Try It 10.5

Weighted alpha is a measure of risk-adjusted performance of stocks over a period of a year. A high positive weighted alpha signifies a stock whose price has risen, while a small positive weighted alpha indicates an unchanged stock price during the time period. Weighted alpha is used to identify companies with strong upward or downward trends. The weighted alpha for the top 30 stocks of banks in the Northeast and in the West as identified by Nasdaq on May 24, 2013 are listed in Table 10.6 and Table 10.7, respectively.

 94.2 75.2 69.6 52 48 41.9 36.4 33.4 31.5 27.6 77.3 71.9 67.5 50.6 46.2 38.4 35.2 33 28.7 26.5 76.3 71.7 56.3 48.7 43.2 37.6 33.7 31.8 28.5 26
Table 10.6 Northeast
 126 70.6 65.2 51.4 45.5 37 33 29.6 23.7 22.6 116.1 70.6 58.2 51.2 43.2 36 31.4 28.7 23.5 21.6 78.2 68.2 55.6 50.3 39 34.1 31 25.3 23.4 21.5
Table 10.7 West

Is there a difference in the weighted alpha of the top 30 stocks of banks in the Northeast and in the West? Test at a 5 percent significance level. Answer the following questions:

1. Is this a test of two means or two proportions?
2. Are the population standard deviations known or unknown?
3. Which distribution do you use to perform the test?
4. What is the random variable?
5. What are the null and alternative hypotheses? Write the null and alternative hypotheses in words and in symbols.
6. Is this test right-, left-, or two-tailed?
7. What is the p-value?
8. Do you reject or not reject the null hypothesis?
9. At the _____ level of significance, from the sample data, there ______ (is/is not) sufficient evidence to conclude that ______.
10. Calculate Cohen’s d and interpret it.
Order a print copy

As an Amazon Associate we earn from qualifying purchases.