Experimental Designs

Question 1

A florist wants to determine if a new additive helps extend the life of cut flowers longer than the original additive does. The florist selects 20 flowers of different types and puts each flower in its own vase with the same amount of water. She positions the vases so they also receive the same exposure to light. She numbers the flowers 1–20, and places these numbers on equal-sized slips of paper. The slips are placed in a hat and mixed thoroughly. A slip is chosen and the corresponding flower receives the new additive. After the hat is shaken, another slip is chosen and the corresponding flower also receives the new additive. This procedure continues until 10 flowers have been assigned to receive the new additive. The remaining 10 flowers receive the original additive.

Revolt · Accepted Answer

A: Yes, each flower is randomly assigned to the treatments.

Question 2

A biology student wants to determine if using a fertilizer would help promote the growth of new babies in spider plants. The student chooses 100 baby spider plants to be used in the study. They all are potted in the same amount and type of soil, given the same amount of water, and exposed to the same amount of light. Half of the plants are randomly assigned to the group that receives fertilizer every three months and the remaining plants are assigned to the group that does not receive fertilizer. After one year, the shoots are counted for each plant. The average number of shoots for the plants receiving fertilizer is compared with the average number of shoots for the plants that did not receive fertilizer.

Revolt · Accepted Answer

A: Yes, the plants were randomly assigned each treatment.

Question 3

A tire company wants to determine if tires made with a new type of tread will last longer than the tires made with the original type of tread. The company has access to 24 different vehicles. The vehicles will be driven for one year and the depth of the remaining tread will be measured. The average depth for the new type of tread will be compared to the average depth of the original type of tread.

Revolt · Accepted Answer

B: There are six vehicles of each vehicle type: sedan, SUV, minivan, and truck. For each type, the vehicles will be numbered 1–6, and a random number generator will be used to pick three unique numbers. These three vehicles will receive tires with the new tread and the other three vehicles in the group will receive tires with the original tread.

Question 4

A shoe company wants to test an updated model of a running shoe to test its wear after one month of running. They recruit 50 people who run on a regular basis to participate in their study. They will have the runners wear the shoes when they run for one month. After one month, the wear on the shoes will be determined using the depth of the tread and flexibility of the toe box. The wear of the new and original models will then be compared.

Revolt · Accepted Answer

D: The 50 subjects are grouped based on running ability. Twenty runners classify themselves as competitive runners and the remaining 30 classify themselves as recreational runners. For the competitive group, the runners’ names are written on equal-sized slips of paper and placed into a hat. The slips are shuffled, and the first 10 runners wear the new model of shoes and the other 10 wear the original model. The same procedure is used to assign the shoes to the recreational group.

Question 5

A tire company wants to determine if tires made with a new type of tread will last longer than tires made with the original type of tread. The company decides to put tires with the new tread on 10 different vehicles and tires with the original tread on 10 different vehicles. "New” is written on one slip of paper and "Original” is written on another slip of paper. The two slips are placed in a bag and thoroughly mixed. A vehicle is selected and a slip is drawn. The corresponding tread of tires is assigned to the vehicle. The slip is put back into the bag and mixed. The next vehicle is selected and a slip is drawn. The corresponding tread is assigned to the vehicle. This procedure continues until all 20 vehicles have been assigned tires of the new or original types of tread.

Revolt · Accepted Answer

C: No, even though randomness is used, the procedure does not ensure that an equal number of vehicles are in both groups.

Question 6

A biology student wants to determine if using a fertilizer would help promote the growth of new babies in spider plants. The student has access to 90 baby spider plants of three varieties: green, variegated, and curly. There are 30 plants of each variety. They all are potted in the same amount and type of soil, given the same amount of water, and exposed to the same amount of light. The numbers 1–30 are written on slips of paper, placed in a hat, and mixed thoroughly. A plant is selected and a slip of paper is drawn. If the slip has the numbers 1–15, then the plant will receive fertilizer. If the slip has the numbers 16–30, the plant will not receive fertilizer. A green spider plant is selected and a slip of paper is drawn. This plant is placed in the treatment group indicated by the number, and the slip is not put back in the bag. The slips are mixed again, the next green spider plant is selected, and a slip is drawn. The plant is placed in the treatment group indicated by the number. This procedure is repeated until all 30 green spider plants are assigned to treatments. The numbered slips are placed back in the bag and this procedure is repeated for the remaining types of spider plants. After one year, the shoots will be counted for each plant.

Revolt · Accepted Answer

C: randomized block design

Question 7

A tire company wants to determine if tires made with a new type of tread will last longer than the tires made with the original type of tread. The company has access to 24 different vehicles. The vehicles will be driven for one year and the depth of the remaining tread will be measured. The average depth for the new type of tread will be compared to the average depth of the original type of tread.

Revolt · Accepted Answer

A: There are six vehicles of each vehicle type: sedan, SUV, minivan, and truck. For each type, three vehicles will be randomly selected to receive tires with the new tread and the other three vehicles will receive tires with the original tread.

Question 8

A shoe company wants to test an updated model of a running shoe on its wear after one month of running. They recruit 50 people who run on a regular basis to participate in the study. Twenty-five runners will receive a pair of the new model and the remaining 25 will receive a pair of the current model. After one month, the wear on the shoes will be determined using the depth of the tread and the flexibility of the toe box. The wear between the new and current models will then be compared.

Revolt · Accepted Answer

A: The subjects are numbered 1–50 and these numbers are put into a random number generator. The first 25 random numbers, ignoring repeats, represent the subjects assigned to the new model group. The remaining 25 subjects will wear the current model.

Question 9

A new cream was developed to reduce the irritation caused by poison ivy. To test the effectiveness, researchers placed an ad online asking for volunteers to participate in the study, and 100 subjects replied. They are informed that one group will receive a new cream and the other group will receive a cream with no active ingredient. The researchers believe that sun exposure may affect the cream’s effectiveness. The subjects are then asked about their outdoor activities, and 42 of the subjects state they spend more than 50% of their day outside. All subjects are exposed to poison ivy and given their cream. They are asked to return in three days and report their level of irritation.

Revolt · Accepted Answer

A: The subjects are grouped based on their outdoor activity. The 42 subjects are randomly assigned to the two treatment groups. The remaining 58 subjects are also randomly assigned to the two treatment groups.

Question 10

A biology student wants to determine if using a fertilizer would help promote growth of new babies in spider plants. He also believes that the type of spider plant may also affect the propagation. The student gets 90 plants, 30 of each spider plant variety: green, variegated, and curly. They all are potted in the same amount and type of soil, given the same amount of water, and exposed to the same amount of light. For each type of plant, the plants with similar sizes are placed in groups of two and labeled with the numbers 1 and 2. The letters A and B are written on equal-sized slips of paper and placed in a bag. The bag is shaken and, for the first group of two, a slip is drawn. If A is drawn, plant 1 will receive fertilizer and plant 2 will not receive fertilizer. If B is drawn, then plant 2 will receive fertilizer and plant 1 will not receive fertilizer. The slip is placed back into the bag and the bag is shaken. This procedure is repeated until all 90 plants have been assigned a treatment. After one year, the shoots will be counted for each plant.

Revolt · Accepted Answer

B: matched pairs design

Experimental Designs Answers

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