Suppose you want to determine whether the brand of laundry
detergent used and the temperature affects the amount of dirt
removed from your laundry. To this end, you buy two different brand
of detergent ( Super and Best) and choose three different
temperature levels (cold, warm, and hot). Then you divide your
laundry randomly into 6r piles of equal size and assign each r=4
piles into the combination of (Super and Best) and (cold,warm, and
hot).
super
best
cold
4,5,6,5...