Dual-energy X-ray absorptiometry (DXA) is a technique for measuring bone health.

Question

Dual-energy X-ray absorptiometry (DXA) is a technique for measuring bone health. One of the most common measures is total body bone mineral content (TBBMC). A highly skilled operator is required to take the measurements. Recently, a new DXA machine was purchased by a research lab, and two operators were trained to take the measurements. TBBMC for eight subjects was measured by both operators. The units are grams (g). A comparison of the means for the two operators provides a check on the training they received and allows us to determine if one of the operators is producing measurements that are consistently higher than the other. Here are the data.

(a)

Take the difference between the TBBMC recorded for Operator 1 and the TBBMC for Operator 2. (Use Operator 1 minus Operator 2. Round your answers to four decimal places.)

x=

s=

Describe the distribution of these differences using words.

1. The distribution is left skewed.

2. The distribution is Normal.    

3. The distribution is uniform.

4. The sample is too small to make judgments about skewness or symmetry.

5. The distribution is right skewed.

(b)

Use a significance test to examine the null hypothesis that the two operators have the same mean. Give the test statistic. (Round your answer to three decimal places.)

t =

Give the degrees of freedom.

Give the P-value. (Round your answer to four decimal places.)

Give your conclusion. (Use the significance level of 5%.)

1. We can reject H0 based on this sample.

2. We cannot reject H0 based on this sample.    

(c)

The sample here is rather small, so we may not have much power to detect differences of interest. Use a 95% confidence interval to provide a range of differences that are compatible with these data. (Round your answers to four decimal places.)

( __, __ )

(d)

The eight subjects used for this comparison were not a random sample. In fact, they were friends of the researchers whose ages and weights were similar to the types of people who would be measured with this DXA machine. Comment on the appropriateness of this procedure for selecting a sample, and discuss any consequences regarding the interpretation of the significance-testing and confidence interval results.

1. The subjects from this sample, test results, and confidence interval are representative of future subjects.

2. The subjects from this sample may be representative of future subjects, but the test results and confidence interval are suspect because this is not a random sample.   

Subject Operator 1 2 3 4 5 6 7 8 1 1.327 1.337 1.076 1.227 0.939 1.005 1.180 1.287 2 1.323 1.322 1.073 1.233 0.934 1.019 1.184 1.304

Explanation / Answer

### by using r command

> a=c(1.327,1.337,1.076,1.227,0.939,1.005,1.180,1.287)
> a
[1] 1.327 1.337 1.076 1.227 0.939 1.005 1.180 1.287
> b=c(1.323,1.322,1.073,1.233,0.934,1.019,1.184,1.304)
> b
[1] 1.323 1.322 1.073 1.233 0.934 1.019 1.184 1.304
> x=a-b
> x
[1] 0.004 0.015 0.003 -0.006 0.005 -0.014 -0.004 -0.017
> xbar=mean(x)
> xbar
[1] -0.00175
> s=sqrt(var(x))
> s
[1] 0.0106066
> hist(x)
> t.test(x,y,paired=T)
Error in t.test.default(x, y, paired = T) : object 'y' not found
> t.test(a,b,paired=T)

Paired t-test

data: a and b
t = -0.46667, df = 7, p-value = 0.6549
alternative hypothesis: true difference in means is not equal to 0
95 percent confidence interval:
-0.010617341 0.007117341
sample estimates:
mean of the differences
-0.00175

a)

xbar=  -0.00175
s= 0.0106066

The sample is too small to make judgments about skewness or symmetry.

b) Null hypothesis: The two operators have the same mean.

Against

Alternative hypothesis: The two operators does not have the same mean.

Here we used paired t test:

Test statistics t=-0.46667

The degrees of fredom are 7

Pvalue is =0.6549

Since P-value is greater than the level of significance. We cannot reject H0 based on this sample.

c) The 95% confidence interval for the difference of two population mean is given by

[-0.0106, 0.0071]

d) The subjects from this sample may be representative of future subjects, but the test results and confidence interval are suspect because this is not a random sample.

Related Questions

Navigate

• Browse (All)
• Browse D
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.