Question

Lazurus Steel Corporation produces iron rods that are supposed to be 36 inches long. The machine that makes these rods does not produce each rod exactly 36 inches long. The lengths of these rods vary slightly. It is known that when the machine is working properly, the mean length of the rods made on these machine is 36 inches. The standard deviation of the lengths of all rods produced on this machine is always equal to .10 inches. The quality control department takes a sample of 40 such rods every week, calculates the mean length of these rods, and makes 99% confidence interval for the population mean. If either the upper limit of this confidence interval is greater than 36.05 inches or the lower limit of this confidence is less than 35.95 inches, the machine is stopped and adjusted. A recent such sample of 40 rods produced a mean length of 36.02 inches. Based on this sample, will you conclude that the machine needs an adjustment? What is the predominant formula you will use to solve the problem?

Answer 1

Answer:

Step-by-step explanation:

We would construct a confidence interval.

Confidence interval is written in the form,

(Sample mean - margin of error, sample mean + margin of error)

The sample mean, x is the point estimate for the population mean.

Since the sample size is large and the population standard deviation is known, we would use the following formula and determine the z score from the normal distribution table.

Margin of error = z × σ/√n

Where

σ = population standard Deviation

n = number of samples

From the information given

x = 36.02 inches

σ = 0.1 inches

n = 40

To determine the z score, we subtract the confidence level from 100% to get α

α = 1 - 0.99 = 0.01

α/2 = 0.01/2 = 0.005

This is the area in each tail. Since we want the area in the middle, it becomes

1 - 0.005 = 0.995

The z score corresponding to the area on the z table is 2.58. Thus, confidence level of 99% is 2.58

Margin of error = 2.58 × 0.1/√40 = 0.041

Confidence interval = 36.02 ± 0.041

The lower end of the confidence interval is

36.02 - 0.041 = 35.979 inches

The upper end of the confidence interval is

36.02 - 0.041 = 36.061 inches

Since 36.061 > 36.05, then we would conclude that the machine needs an adjustment.

The formula used is

Confidence interval =

Sample mean ± z × σ/√n