Absorbency was tested in one of two ways, speed of absorption and volume of absorption. When testing speed, the more rapidly the water moves through the towel, the more absorbent the towel. When testing volume, the more water a towel will hold, the more absorbent the towel.
(5) Place a paper towel in the embroidery hoop. Draw a circle 2 inches in diameter in the center of the towel.
a) Place drops of water in the center of the towel. Count the number of drops added until the water spreads out to fill the circle. The towels requiring fewer drops are the more absorbent towels.
b) Place 10 drops of water in the center of the circle. Measure the time it takes for water to spread out and fill the circle. The towels with shorter times are the more absorbent towels.
(6) Cut towels into strips 1 inch wide and 6 inches long. Draw marks 1 inch from each end with an overhead pen. Fill a cup with water. Hold each strip vertically with tweezers with the bottom end one inch in the water. Measure the time it takes for the water to travel from bottom mark (at the top of the water) to the top mark. This measures how quickly the towel absorbs water. The faster the water moves through the towel, the more absorbent the towel.
(7) Put one cup of water in a measuring cup. Submerge a towel in the water for 5 seconds. Remove the towel. Measure the amount of water remaining in the cup to determine the amount of water removed from the measuring cup. The greater the amount of water removed from the cup, the more absorbent the towel. The size of the paper towels varies as does the cost. This experiment can be modified to compare
a) a single sheet, since that’s what people use
b) a fixed area, either by a fixed area of towel or by using a whole towel and making measurement on a per unit area basis
c) fixed price, either by using a section that costs 0.5 cents or by making the measurement on a fixed cost basis. NCSSM Statistics Leadership
Institute Notes Experimental Design 65
(8) Put one cup of water in a measuring cup. Submerge a towel in the water for 5 seconds. Remove the towel. Let the towel drip for 30 seconds back into the measuring cup. Measure the amount of water removed from the cup to determine the amount of water retained in the towel. The greater the amount of water retained in the towel, the more absorbent the towel. This experiment can be modified as in Experiment (7). Design of Experiments
All of these suggested experiments can be carried out using either a completely randomized design or a randomized complete block design. If the experimenters believe that they will become more skilled (or less skilled) at performing the experiment and making the measures as the experiment progresses, they should use a randomized complete block design, using each brand of towel in random order for each block. If no learning effect is expected, a completely randomized design should be used. Many of the following experiments designed by the participants utilize a randomized complete block design, using the trial number as the blocking variable. In the analysis it is clear that this blocking variable adds little to the experiment. A completely randomized design would give more power in most of these situations.
Note: Studies of a single factor are generally one of two types. In the first, a random sample is drawn from the population of interest and the levels of the factor are randomly assigned to these experimental units. In the second, random samples are taken from each of k levels of the factor. The paper towel studies are examples of this second type. It is not possible to randomly assign a paper towel to be “Bounty” or any other brand of paper towels. Instead, we took a random sample from each type of paper towel. Some textbooks refer to only the first type of study in which treatments are randomly assigned to experimental units as having a completely randomized design. Here, we use the term for both types as in Inman (1994, p. 691). However, the types of inference that can be drawn from each differs (see Figure and related discussion on page 12).NCSSM Statistics Leadership Institute Notes Experimental Design 66
Results of Participant Experiments
Experiment # 1a
Experimental Protocol: Place a paper towel in the embroidery hoop to maintain a constant tension. Add one tablespoon of water to the center and wait 30 seconds. Then gently place a 50 gram weight in the center of the hoop. Wait 3 seconds and place another weight on top until the towel breaks. Record the weight on the paper towel before the towel broke.