The degenerative disease osteoarthritis most frequently affects weight-bearing joints such as the knee. The article "Evidence of Mechanical Load Redistribution at the Knee Joint in the Elderly When Ascending Stairs and Ramps" (Annals of Biomed. Engr., 2008: 467-476) presented the following summary data on stance duration (ms) for samples of both older and younger adults.

Assume that both stance duration distributions are normal.
a. Calculate and interpret a 99% CI for true average stance duration among elderly individuals.
b. Carry out a test of hypotheses at significance level .05 to decide whether true average stance duration is larger among elderly individuals than among younger individuals.

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Sample Mean 801 780 Age Sample Size Sample SD Older Younger 28 16 72