Experimental Design

1. Experimental Design Determining Independent and Dependent Variables

2. Variables

3. Independent Variable • That factor which is measured, manipulated, or selected by the experimenter to determine its relationship to an observed phenomenon. • This is the thing the researcher changes or manipulates in the experiment. • In an experiment, we test whether the independent causes the dependent.

4. Dependent Variable • That factor which is observed and measured to determine the effect of the independent variable, i.e., that factor that appears , disappears or varies as the experimenter introduces, removes, or varies the independent variable. • It depends in the independent variable • In an experiment, this is the outcome we are trying to change by manipulating the independent variable.

5. Control Variables These are the things that stayed the same throughout the experiment. The control variable is the “control” base from which you can see what changes in the experiment.

6. Extraneous Variables Extraneous variables are the extra variables that were not part of the experiment but that could have affected the experiment data and results. Typically these include: measurement error, researcher error, weather, etc…

7. Example 1 • Dr. Imanut wants to examine whether a new drug increases the maze running • performance of older rats.  Just like aging humans, older rats show signs of • poorer memory for new things.  Dr. Imanut teaches two groups of older rats to • find a piece of tasty rat chow in the maze.  One group of rats is given the new • drug while they are learning the maze.  The second group is not given the • drug.  One week after having learned the maze he retests the rats and records • how long it takes them to find the rat chow. • What is the independent variable? • age of the rats. • b) type of maze. • c) length of time it took the rats to run the maze. • d) presence or absence of the new drug. • What is the dependent variable?  • age of the rats. • b) type of maze. • c) length of time it took the rats to run the maze. • d) presence or absence of the new drug.

8. Answer: Example 1 • D - Independent variable was the presence or absence of the drug.  This was the variable being manipulated by the researcher. • C - Dependent variable was the length of time it took the rats to remember where the rat chow was after one week.  This was the measure of the subjects' response.

9. Example 2 A researcher wanted to study the effects of sleep deprivation on physical coordination.  The researcher selected 25 year-old male college students and deprived some of the subjects to either 24, 36, or 45 hours of sleep. What is the independent variable? a) the length of time the subjects were deprived of sleep. b) the age of the subjects. c) the gender of the subjects. d) the physical coordination skills of the subjects. What is the dependent variable?  a) the length of time the subjects were deprived of sleep. b) the age of the subjects. c) the gender of the subjects. d) the physical coordination skills of the subjects.

10. Answer - Example 2 • A - Independent variable was the length of time the subjects were sleep deprived. • D - Dependent variable was the physical coordination skills of the subjects.

11. Example 3 • A researcher wanted to know whether the number of people • present would influence subjects' judgments on a simple • perceptual task.  In each case the other members of the group • gave an incorrect answer.  The researcher then noted whether • the subject conformed to the group decision. • What is the independent variable? • the number of people in the group. • b) whether the group members gave the correct or incorrect answer. • c) whether the subjects conformed with the group. • d) the type of perceptual task. • What is the dependent variable?  • the number of people in the group. • b) whether the group members gave the correct or incorrect answer. • c) whether the subjects conformed with the group. • d) the type of perceptual task.

12. Answer - Example 3 • A - Independent variable was the number of people in the group. • C - Dependent variable was whether the subjects conformed with the group.

13. Example 4 • An investigator had 60 subjects watch a videotaped • reenactment of a bank robbery.   Half of the subjects were • asked by a police investigator to recall the event, while the • remaining subjects were interviewed by a police investigator • while they were hypnotized. • What is the independent variable? • whether a police investigator was used. • b) whether subjects were hypnotized. • c) how much subjects recalled. • d) what subjects watched. • What is the dependent variable?  • whether a police investigator was used. • b) whether subjects were hypnotized. • c) how much subjects recalled. • d) what subjects watched.

14. Answer - Example 4 • B - Independent variable was whether the subjects were hypnotized. • C - Dependent variable was how much subjects recalled.

15. How much water flows through a faucet at different openings? • Water faucet opening (closed, half open, fully open) • Independent Variable • Amount of water flowing measured in liters per minute • Dependent Variable

16. Does heating a cup of water allow it to dissolve more sugar? • Amount of sugar that dissolves completely measured in grams • Dependent Variable • Temperature of the water measured in degrees Centigrade • Independent Variable

17. Does fertilizer make a plant grow bigger? • Growth of the plant measured by its height • Dependent Variable • Amount of fertilizer measured in grams • Independent Variable

18. Does an electric motor turn faster if you increase the voltage? • Voltage of the electricity measured in volts • Independent Variable • Speed of rotation measured in revolutions per minute (RPMs) • Dependent Variable

19. How fast does a candle burn? • Height of candle measured in centimeters at regular intervals of time (for example, every five minutes) • Dependent Variable • Time measured in minutes • Independent Variable