In: Statistics and Probability
A springmaker is supplying helical coil springs. The customer has specified that the springs must be 10 +/- 1 N/m in stiffness (i.e. this is the specification for the spring). Testing of springs indicates that their stiffness follows a normal distribution.
a) Data from inspection has shown that 8.1 percent of parts are scrapped on the low end (i.e. k < 9 N/m) and 5.5 percent are scrapped on the high end (i.e. k > 11 N/m). What is the mean value and standard deviation of the population of parts that has been inspected?
b) What should my specification limits be for this populations of parts to ensure that only 2.5 percent of parts are scrapped on each of the high and low end of the specification (i.e. 2.5 percent of parts will be above the his specification limit and 2.5 percent of parts will be below the lower specification limit).
c) If I take one part, measure it, and get a stiffness of 10.8 N/m. What percentage of parts have a lower stiffness value than this part.