Suppose you are a quality control manager at a manufacturing company that produces light bulbs. The company has recently made changes to the manufacturing process, and you want to investigate whether these changes have had a significant impact on the average lifespan of the bulbs. You collect a random sample of 30 light bulbs produced under the new process and find that the sample mean lifespan is 1200 hours with a sample standard deviation of 100 hours. Additionally, you collect a random sample of 35 light bulbs produced under the old process and find that the sample mean lifespan is 1180 hours with a sample standard deviation of 90 hours. Assuming that the population standard deviations are unknown but equal, test at a 5% significance level that the average lifespan of the light bulb has changed with the new manufacturing process.    NOT FOR MARKS, just a book sample question

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 + ...