Ps alleged defects in Kenmore-brand Sears washing machines sold in overlapping periods beginning in 2001 and 2004. One class action complains of a defect that causes mold (the mold claim), the other of a defect that stops the machine inopportunely (the control unit claim). The mold claim pertains to all Kenmore-brand frontloading washing machines manufactured by Whirlpool Corporation and sold by Sears since 2001. The low volume of water used in these machines and the low temperature of the water, compared to the volume and temperature of the water in the traditional top-loading machine, means that the machines don't clean themselves adequately and as a result biofilm-a mass of microbes-forms in the machine's drum (where the washing occurs) and creates mold, which emits bad odors. Household cleaners do not eliminate the biofilm, the mold, or the odors. Roughly 200,000 of these Kenmore-brand machines are sold each year, and there have been many thousands of complaints of bad odors by the owners. D contends that a number of design modifications as a result of which different models are differently defective and some perhaps not at all, and therefore common questions of fact concerning the mold problem and its consequences do not predominate over individual questions of fact. The district court denied certification of the class complaining about the defect that causes mold and granted certification of the class complaining about the defect that causes the sudden stoppage. The mold class appealed. D also appealed contending the certification of the control unit claim was erroneous.