2. Blocking and Matched Pairs

Task: Cooking
      Each group will choose a food item they want to run a taste test experiment with.  Then, the group needs to change one thing about the item to see if people prefer one version or the other.  Examples include:
      • Venison with two different types of seasoning
      • Uncolored eggs vs. green eggs
      • Pepsi Cola vs. Coca Cola
      • Butter vs. margarine on toast.
      Design an experiment that allows you to find causation by eliminating lurking variables.  For foods with different appearances, this might mean blindfolding the consumer or removing labels.  (For example, your study may find that people like the taste of butter more than margarine, and you made sure they didn't know which one they were eating during the test).  Choose a way to measure response.  This could include:
      • Rate the item 1 to 5 (2 group quantitative)
      • If matched pairs, rate how much better one item is than the other -2 to +2 (matched pairs quantitative)
      • If matched pairs, choose which one you prefer (matched pairs categorical)

      Mastery Quiz Prep

      For the problems below, you need to prove each of the statements with an experiment.  Answer a-b for each:
      • a) Could you use a matched pairs design with this study? If so, what would the matched pairs be? How would you decide when each treatment is received / who receives each treatment from the pair? 
      • b) Besides what you used for matched pairs, is there a logical variable to block by? How many experimental units would fall in each block? 
      1. Premium gasoline (89 octane) gives cars better gas mileage than regular gasoline (87 octane).

      2. A server who suggests the most popular appetizer to customers at Restaurant A will make more appetizer sales than a server who asks “can I start you out with an appetizer?” 

      3. A new hip replacement procedure, when compared to the existing common procedure and to no procedure at all, will lead to more natural walking (as rated 1 to 10 by physical therapists) 2 months after surgery. 

      4. Taking a recently developed pill each day will reduce the number of headaches experienced over the next 3 months. 

      5. A person who smokes is more likely to get lung cancer than someone who does not smoke. (Hint: if your experiment is nearly impossible to do well with humans, use rats). 

      Block the subjects in the experiment below by their math grades.

      6.    An education study wants to compare student performance in a problem-set-only course and a course infused with multiple large projects. The students in the table are available for the study:

       Name Last math grade
       Bill 67
       Mary 74
       Suzy 83
       James 92
       Josh 89
       T.J. 93
       Missy 78
       Kirsten 98

      >> 13 and 15 on attached PDF below

      Free Response Prep
      • In order to test people's willingness to obey an authority figure against their conscience, the psychologist Stanley Milgram used scientists in white lab coats to ask subjects to provide an electric shock to a "student" who incorrectly answered questions on the other side of a curtain.  The subjects were told that the shock would help the student learn better and motivate correct answers.  In reality, the person behind the curtain was not actually being shocked, but was just an actor who was helping with the experiment.  The result of the study was that most people would apply the highest level shock, despite their reservations, because the scientist in the lab coat told them it was necessary.
      • Additional information of the scenario: YouTubeWikipedia
        • Question 1: Discuss the benefits of this research.  Justify why the experiment had to be conducted the way it was.
        • Question 2: Discuss the ethical problems with this kind of research.
        • Question 3: Explain what an IRB is and why they exist today for human-subject research.

      Practice solutions

      1. Gasoline
      • a) Yes -- the same car could be tested first with one type of gas, then the other, and compared.  You would randomly decide which type of gas is used first.
      • b) You could form blocks by type of car.  There would be 2 similar cars per block (each block would have one using regular and one using premium gas).
      2. Appetizers
      • a) Probably not -- you can only ask the same customer once if they want an appetizer.  It would be hard to know when the same customer comes back to be tested with the other treatment.
      • b) You could block by time of day, family size, or server (or by all of these).  Either way, there should be only 2 subjects (tables) per block -- one for each treatment.
      3. Hip replacement
      • a) If the patient had issues with both hips, you could compare two of the treatments in the same person.  This might not be such a great idea though and might confound two treatments when the therapist judges the walking.
      • b) If you blocked by prior walking ability, then each treatment group would be more likely to be about equal.
      4. Headaches
      • a) You could do the study at different times for every patient -- 3 months of one, a break, and 3 months on the other.  If the patient can sense the difference between drugs, this could be a problem.
      • b) You could block subjects by number of headaches they normally experience so that both high and low headache folks are in each treatment group
      5. Smoking
      • a) Identical rat twins?  Probably not.
      • b) Perhaps size of rat or general health of rat.
      6. Blocking
      Since the study is comparing 2 treatments, students should be placed in blocks of 2.  The 2 highest scores, then the next 2 highest, and so forth should be in a block.  So:
      [Kirsten, T.J.], [James, Josh], [Suzy, Missy], [Mary, Bill]
      The reason you might want to do this is because it makes it less likely that one group starts with all the stronger students -- it minimizes lurking variables (confounding factors).

      13-16: see attached PDF file


          Andy Pethan,
          Nov 12, 2015, 9:50 PM
          Andy Pethan,
          Nov 12, 2015, 9:50 PM