Blocking A block is a group of subjects that are similar in some way that is expected to affect the response to the treatments. In a block design, the random assignment of units to treatments is carried out separately within each block.
More on blocking Unless the link is obvious, you must justify the reason for blocking on any variable! The difference between randomization and blocking: You block to control for the variables you know about that might influence the response You randomize to control for the variables you do NOT know about. Blocks allow us to draw separate conclusions about each block.
A way to control the placebo effect! Avoids unconscious bias Weakness of experiments: Lack of realism (setting, subjects, or treatments may not realistically duplicate the conditions we want to study).
Matched Pairs Experiment - options 1)Matching individuals based on a set of characteristics such as gender, age, income, level of education, and religion, then placing them into control and treatment groups. (the assumption is that the two groups should be similar in terms of the factors on which they were matched. 2)Measure the same individuals at two different times.
Example 1 A medical study of heart surgery investigates the effect of a drug called a beta-blocker on the pulse rate of the patient during surgery. The pulse rate will be measured at a specific point during the operation. The investigators will use as subjects 20 patients facing heart surgery. You have a list of these patients, numbered 1 to 20, in alphabetical order. a)Outline a well-designed experiment for this (list and label everything) b)Use the random digit table starting at line 125 to carry out the randomization required by your design and list the results.
Example 2 You’re interested in whether white Americans’ attitudes about prejudice towards African- Americans change after they learn more about the history of African-Americans. You identify a diverse sample of whites that contains men and women of varying ages, incomes, religions, and levels of education. You decide to use a documentary as a way of teaching individuals more about the history of the African-American experience. How would you use each type of matched pairs experiment to examine your hypothesis on attitudes about prejudice? Design with a partner and share out.
Example 3 Professor Hampton conducts an experiment on the possible effects of room temperature on the speed at which a machine produces rubber snakes. He identifies 100 machines and sets up the following design: 50 machines are placed in a “warm” room (85 degrees) 30 of the machines in the “warm” room are placed near the windows 20 of the machines in the “warm room are placed away from the windows 50 machines are placed in a “cold” room (55 degrees) 15 of the machines in the “cold” room are placed near the windows 35 of the machines in the “cold” room are placed away from the windows a)What are the factors in this experiment? What are the levels of each of these factors? b)How many treatment conditions are there in this experiment? What are the treatment conditions? How many machines are in each treatment condition? c)What is the response variable?