Question: The Robertson square drive screw was invented in 1908 but
The Robertson square drive screw was invented in 1908, but it has gained in popularity with American woodworkers and home craftspeople only within the last 10 years. The advantages of square drives over conventional screws is indeed remarkable—most notably greater strength, increased holding power, and reduced driving resistance and “cam-out.” Strength test results published in McFeely’s 2005 catalog revealed that the no. 8 Robertson square drive flat head steel screws fail only after an average of 46 inch-pounds of torque is applied, a strength nearly 50% greater than that of the more common slotted- or Phillips-head wood screw. Suppose an independent testing laboratory randomly selects 22 square drive flat head steel screws from a box of 1000 screws and obtains a mean failure torque of 45.2 inch-pounds and a standard deviation of 5.1 inch-pounds. Estimate with 95% confidence the mean failure torque of the no. 8 wood screws based on the study by the independent laboratory. Specify the population parameter of interest, the criteria, the sample evidence, and the interval limits.
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