The circumference of a particular species of tree was thought to be different depending on certain geographical factors. An SRS of 60 trees is selected in Sequoyah National Forest, where it is found that = 6 meters with a standard deviation = 3 meters. A second SRS of size 40 is independently selected in Yellowstone National Forest, and it is found that = 4 meters with a standard deviation = 2 meters. Let μ1 and μ2 represent, respectively, the true mean circumferences of this particular species of tree. Assume the standard deviations of the two populations are equal. A 95% confidence interval for μ1 – μ2 is

Definition Definition Measure of central tendency that is the average of a given data set. The mean value is evaluated as the quotient of the sum of all observations by the sample size. The mean, in contrast to a median, is affected by extreme values. Very large or very small values can distract the mean from the center of the data. Arithmetic mean: The most common type of mean is the arithmetic mean. It is evaluated using the formula: μ = 1 N ∑ i = 1 N x i Other types of means are the geometric mean, logarithmic mean, and harmonic mean. Geometric mean: The nth root of the product of n observations from a data set is defined as the geometric mean of the set: G = x 1 x 2 ... x n n Logarithmic mean: The difference of the natural logarithms of the two numbers, divided by the difference between the numbers is the logarithmic mean of the two numbers. The logarithmic mean is used particularly in heat transfer and mass transfer. ln x 2 − ln x 1 x 2 − x 1 Harmonic mean: The inverse of the arithmetic mean of the inverses of all the numbers in a data set is the harmonic mean of the data. 1 1 x 1 + 1 x 2 + ...