An aspirin manufacturer has designed the childproof cap on its bottles so that it must be pressed down before it can be turned and removed. A consumer magazine has randomly purchased 15 bottles of the company's aspirin for analysis in a feature article on pain relievers and their packaging.
Among the analyses is a test of how much force is required to depress the cap on each bottle. The standard deviation for the bottles made by this manufacturer is found to be 7.2 grams.
From the magazine's findings, and assuming a normal distribution, determine the 95% confidence interval for the population standard deviation of the force required to depress the firm's aspirin bottle caps.