In: Math
David E. Brown is an expert in wildlife conservation. In his book The Wolf in the Southwest: The Making of an Endangered Species (University of Arizona Press), he records the following weights of adult grey wolves from two regions in Old Mexico.
Chihuahua region: x1 variable in pounds
86 | 75 | 91 | 70 | 79 |
80 | 68 | 71 | 74 | 64 |
Durango region: x2 variable in pounds
68 | 72 | 79 | 68 | 77 | 89 | 62 | 55 | 68 |
68 | 59 | 63 | 66 | 58 | 54 | 71 | 59 | 67 |
(a) Use a calculator with mean and standard deviation keys to calculate x1, s1, x2, and s2. (Use 2 decimal places.)
x1 | |
s1 | |
x2 | |
s2 |
(b) Let μ1 be the mean weight of the population
of all grey wolves in the Chihuahua region. Let
μ2 be the mean weight of the population of all
grey wolves in the Durango region. Find a 99% confidence interval
for μ1 – μ2. (Use 2 decimal
places.)
lower limit | |
upper limit |
(c) Examine the confidence interval and explain what it means in the context of this problem. Does the interval consist of numbers that are all positive? all negative? of different signs? At the 99% level of confidence, what can you say about the comparison of the average weight of grey wolves in the Chihuahua region with the average weight of grey wolves in the Durango region?
Because the interval contains only positive numbers, we can say that the mean weight of grey wolves is greater in the Chihuahua region.Because the interval contains only negative numbers, we can say that the mean weight of grey wolves is greater in the Durango region. Because the interval contains both positive and negative numbers, we can not say that the mean weight of grey wolves is greater in the Chihuahua region.We can not make any conclusions using this confidence interval.
a)
b) The confidence interval for difference of means is
where
are the t-value for the desired level of confidence and estimated
standard error of difference between the means.
For our case, for
confidence and
degrees of freedom can be looked up to be equal to
.
Estimated standard error can be calculated as
c) The range of the confidence interval comprises only of positive numbers. This suggests that the average weight of grey wolves is greater in the Chihuahua region as compared to those in the Durango region.