PS295 Lecture 4: Manipulating Independent Variables
Manipulating Independent Variables
Considerations when manipulating a variable:
1) How do you ONLY manipulate the IV?
2) How many levels of the IV should you have?
3) How can you strengthen the manipulation?
4) What type of manipulation will you use?
Working example:
A researcher is interested in effect of food restriction on wheel running in rats. Based on previous
researc, she believes that as food consumption decreases, activity increases. She is in the process of
designing her sudy, which includes operational defining her variables.
IV: food restriction
DV: wheel running
Hypothesis: as IV decreases, DV increases
-activity based anorexia type of model
-suugests that human anorexia can have a biological component to it
1) How do you only manipulate the independent variable (food restriction)?
-the goal is manipulate the IV while keeping everything else constant
ex using 3 different groups:
Level 1: reduce food intake from 25g/day (how much normal adult rat consumes/day) to 20g/day (low
condition)
Level 2: reduce food intake from 25g to 15g/day (moderate condition)
Level 3: reduce food intake from 25g to 10g/day (high condition)
-all rats get the same other conditions; ex pet the same amount/at the same time a day,
lights/environment remains the same, all that changes is food consumption
2) How many levels of the IV/how many groups should you have?
-need at least 2
-levels of the IV are systematically varied by the researcher
-rule of thumb: use as many lavels as needed to adequately test the hypothesis
The number of levels chosen is influenced by:
A) past research
B) complexity of expected results
-researcher believes relationship among variables is curvilinear rather than linear
-there will be an optimal level when running is highest
-need at least 3 groups
-2 groups allow for linear relationship
C) how specific the researcher wants the conclusions to be
D) Availability of participants
E) Costs
3) How Can you Strengthen the Manipulation?
-consider how different the levels are from each other
Levels closer together:
Document Summary
A researcher is interested in effect of food restriction on wheel running in rats. Based on previous researc, she believes that as food consumption decreases, activity increases. She is in the process of designing her sudy, which includes operational defining her variables. The goal is manipulate the iv while keeping everything else constant ex using 3 different groups: Level 1: reduce food intake from 25g/day (how much normal adult rat consumes/day) to 20g/day (low condition) Level 2: reduce food intake from 25g to 15g/day (moderate condition) Level 3: reduce food intake from 25g to 10g/day (high condition) Levels of the iv are systematically varied by the researcher. Rule of thumb: use as many lavels as needed to adequately test the hypothesis. The number of levels chosen is influenced by: past research, complexity of expected results. Researcher believes relationship among variables is curvilinear rather than linear. There will be an optimal level when running is highest.
