Engineers tested the shear strength of a new brand of rivets to ensure that they were at least as strong as the brand already approved for use building a new bridge. The engineers randomly selected 200 approved-brand rivets and 200 new-brand rivets and used a hydraulic press to apply shear forces on the rivets. The force necessary for failure was recorded. Determine the degrees of freedom for a two-mean hypothesis test for population variances assumed unequal (nonpooled estimate of the standard deviation) to test whether the new rivets are at least as strong