An experiment is designed to compare the corrosion resistance of three metals, steel,copper and zinc, in a harsh environment. Identical units, each made out of one of the three metals, are available for testing. Six units of each metal product were assigned at random to two groups of three each. The rest group of units had weights measured before and after an exposure period of 30 hours and the second group had weights measured before and after 60 hours exposure. The data (in terms of percentage weight loss) were recorded as follows.