1. How to Design an Experiment: Part II
Module 4
Experimental psychology guided-inquiry learning
Module 4: How to Design an Experiment
©2012, Dr. A. Geliebter & Dr. B. Rumain, Touro College & University System

2. Levels of an Independent Variable
In the experiment we just did in Part I, we say there are 3 levels of the independent variable corresponding to the three dosages. We can diagram it as follows: Independent variable = dosage of the drug “Elate” If there are 3 possibilities, then there are 3 levels of the independent variable corresponding to the 3 dosages
3 levels of the independent variable
(Dosage)
Experimental Group I
Experimental Group II
Control Group
750 mg
1200 mg
0 mg

3. Levels of an Independent Variable
If there are 2 possibilities, Experimental Group I with 750 mg, and the control group (with 0 mg), then we would say there are 2 levels of the independent variable (corresponding to the two dosages).
2 levels of the independent variable
(Dosage)
Experimental Group I
Control Group
750 mg
0 mg

4. Levels of an Independent Variable
In any experiment, there MUST be at least two levels of the independent variable. WHY?

5. How do we decide how to measure the dependent variable?
We have to decide how we are going to measure depression. Another way of saying this is that we have to decide how we are going to operationalize depression. By “operationalize” we mean coming up with a standard way to measure depression, so we all have a uniform measurement tool. One reason for this is to enable others to replicate the study using the same measurement tool.

6. Thought Questions
5) How would you operationalize depression here in this study? Remember, operationalize means to standardize the meaning of a measure or procedure so the study can be replicated. One possibility is that you could use a Depression Inventory, like the Beck Depression Inventory. The Beck Depression Inventory is a questionnaire in which people answer questions rating themselves on how depressed they feel. In this case, the dependent variable is depression as operationalized by scores on the Depression Inventory.

7. Alternatively, you could have a clinician (psychiatrist, psychologist, or psychiatric social worker) conduct a structured clinical interview with each patient and make an assessment of his/her degree of depression. In this case, depression is operationalized by a rating given at the end of a diagnostic interview. You could also do both things, namely use a score on a Depression Inventory and a rating after a diagnostic interview. If you used both measures, then you’d have two dependent variables or dependent measures.

8. Beck Depression Inventory (BDI)
Developed by Beck, Steer & Brown
Assesses the intensity of depressive symptoms
5-10 minutes to administer
Highly reliable regardless of the population tested
21 multiple-choice questions

9. Beck Depression Inventory (BDI)
Sample questions: I do not feel sad. 1 - I feel sad 2 - I am sad all the time and I can't snap out of it. 3 - I am so sad and unhappy that I can't stand it I am not particularly discouraged about the future. 1 - I feel discouraged about the future. 2 - I feel I have nothing to look forward to. 3 - I feel the future is hopeless and that things cannot improve I do not feel like a failure. 1 - I feel I have failed more than the average person. 2 - As I look back on my life, all I can see is a lot of failures. 3 - I feel I am a complete failure as a person.

10. Beck Depression Inventory (BDI)
The remaining questions can be found at: modules/webimages/Beck_Depression_Inventory.pdf The questions from the Beck Depression Inventory have also been copied to the following slides, for your convenience. Print out the Beck Depression Inventory and complete it. Score your answers, and see where your total score translates in terms of levels of depression. Please do not share your total score or depression level with your classmates.

11. Beck Depression Inventory (BDI)
This depression inventory can be self-scored. The scoring scale is at the end of the questionnaire.

12. Beck Depression Inventory (BDI)

13. Beck Depression Inventory (BDI)

14. Beck Depression Inventory (BDI)

15. Beck Depression Inventory (BDI)

16. Beck Depression Inventory (BDI)

17. Beck Depression Inventory (BDI)
Scoring: Each answer is scored on a scale of 0 to 3 and then all scores are added together to produce a total score. The total score is an indicator of an individual’s level of depression.
Total Score
Level of Depression
1-10
These ups and downs are considered normal
11-16
Mild mood disturbance
17-20
Borderline clinical depression
21-30
Moderate depression
31-40
Severe depression
Over 40
Extreme depression

18. Could use both measures = 2 dependent variables
Beck Depression Inventory
- Patient self-rating
Clinical Interview
- Clinician rating

19. Thought Question
6) If you're going to have a clinician score each patient on his/her level of depression, can that clinician know what medication that patient is receiving? Why? No, the clinician can’t know because that might influence his rating. For example, if a psychiatrist knows a patient is receiving an antidepressant, he might be biased to rate him as less depressed than a patient receiving a placebo.

20. How do we decide how to measure the dependent variable?
A study where the patient doesn’t know whether he’s receiving a placebo or a true medicine is called blind. One in which the patient and the clinician doing the ratings don’t know which patient is receiving what is called double blind.

21. Thought Question
7) How do you get people to participate in a study where they might be getting a placebo rather than true medication? Do you lie to them? No. Before an individual participates in a study, s/he must sign an Informed Consent Form.

22. 8) What would you tell them
8) What would you tell them? You must say something like the following: This is an experimental study to assess the effects of a certain medication. Some of you will receive the medication and some of you will receive a placebo, a sugar-coated pill. We cannot tell you who will get what. We do not even know whether the medication works or what its side effects are. Your participation in this study will help us determine whether the medication is helpful. You are free to withdraw from the study at any time. If the study is being conducted at a hospital/clinic and the patient is receiving services/treatment from the facility, you should also state in the Informed Consent: “If you decide to withdraw, this will not affect your treatment or your relationship with your doctor in any way.”

23. Thought Question
9) Can you have more than one independent variable and more than one dependent variable and how would you do this? A study where there is more than one independent variable and/or more than one dependent variable is called multivariate. The study above with two dependent variables is a multivariate study.

24. Thought Questions
10) To review, what were the 2 dependent variables in the study we were just talking about? One dependent measure was depression as measured by a score on the Beck Depression Inventory. The second dependent measure (or variable) was depression as measured by a rating given at the end of a diagnostic interview with a psychiatrist or psychologist.

25. Multiple Independent Variables
Let’s consider now having more than one independent variable. If one independent variable is dosage, another might be, let’s say, amount of exercise if you thought that exercise also had an effect on a person’s feeling depressed. If you also wanted to look at the effect of exercise in addition to the effect of medication, then you’d have 2 independent variables: (1) dosage of medication, and (2) exercise. Then for the “exercise” variable, you might have 2 levels of it: One level could be a “yes exercise” condition and the other level could be a “no exercise” condition.
2 independent variables 

26. Multiple Independent Variables
Dosage
3 levels:
750mg (experimental group #1)
1200mg (experimental group #2)
0mg (control group)
Independent variable #2:
Exercise
2 levels:
“Yes exercise”
“No exercise”
So, based on the above, how many groups of subjects are there?

27. Multiple Independent Variables
In all, there are 6 groups of subjects in this experiment. How do we know this?

28. Multiple Independent Variables
We multiply the 3 levels of dosage by the 2 levels of exercise, to get the 6 groups of subjects, or 6 conditions. We can also see this by constructing a Latin Square, as follows: Fill in what goes into each of the 6 boxes (the 6 conditions).
Independent Variable #1: Dosage
750mg
1200mg
0mg
Yes No
Independent
Variable #2:
Exercise

29. Multiple Independent Variables
If your Latin Square looks like the one below, then you are correct.
Independent Variable #1: Dosage
750mg
1200mg
0mg
Yes
Group 1:
750mg and Yes Exercise
Group 2: 1200mg and Yes Exercise
Group 3:
0mg and
Yes Exercise No
Group 4:
750mg and No Exercise
Group 5: 1200mg and No Exercise
Group 6:
No Exercise
Independent
Variable #2:
Exercise