1. Observational Studies and Experiments
Chapter 13
Observational Studies and Experiments

2. Observational Study Experiment
Researchers don’t assign choices. They only observe what is already happening.
Researchers manipulate factors. They create a treatment and compare the responses.

3. Observational Study vs Experiment
Video Clip –
Observational Study vs Experiment

4. Observational Study or Experiment?
Over a 4-month period, among 30 people with bipolar disorder, patients who were given a high dose of omega-3 fats from fish oil improved more than those given a placebo.
The leg muscles of men aged 60 to 75 were 50% to 80% stronger after they participated in a 16-week high-intensity resistance training program twice a week.
EXPERIMENT
OBSERVATIONAL STUDY

5. Observational Study or Experiment?
Among a group of disabled women aged 65 and older who were tracked for several years, those who had vitamin B12 deficiency were twice as likely to suffer severe depression as those who did not.
Some gardeners prefer to use nonchemical methods to control insect pests in their gardens. Researchers have designed two kinds of traps and want to know which design is more effective. They randomly choose 10 locations in a garden and place one of each type of trap in each location. After a week, they count the number of bugs in each trap.
OBSERVATIONAL STUDY
EXPERIMENT

6. Observational Study Retrospective: Prospective:
Subjects are selected and then their past conditions are observed
Prospective:
Subjects are followed to observe future outcomes

7. Experimental Design:
Control Make conditions as similar as possible for all treatment groups
Randomize Experimental units (or subjects) need to be assigned to treatments at random
Replicate The outcome of one subject is not enough to be called data
Block (if necessary) Group similar individuals together and then randomize within these blocks

8. Experiment Random Allocation Group 1 Group 2 Treatment 1 Treatment 2
Compare

9. A Step-by-Step Experiment
Plan: State what you want to know.
I want to know whether tomato plants grown with OptiGro yield jucier, tastier tomatoes than plants raised in otherwise similar circumstances but without the fertilizer.

10. Response: Specify the response variable
A Step-by-Step Experiment
Response: Specify the response variable
I’ll evaluate the juiciness and taste of the tomatoes by asking a panel of judges to rate them on a scale from 1 to 7 in juiciness and in taste.

11. Treatments: Specify the factor levels and the treatments
A Step-by-Step Experiment
Treatments: Specify the factor levels and the treatments
The factor is fertilizer (OptiGro). I’ll grow tomatoes at three different factor levels: some with no fertilizer, some with half the specified amount of OptiGro, and some with the full dose of OptiGro. These are the three treatments.

12. Experimental Units: Specify the experimental units
A Step-by-Step Experiment
Experimental Units: Specify the experimental units
I’ll obtain 24 tomato plants of the same variety from a local garden store.

13. Experimental Design: Control
A Step-by-Step Experiment
Experimental Design: Control
I’ll locate the farm plots near each other so that the plants get similar amounts of sun and rain and experience similar temperatures. I will weed the plots equally and otherwise treat the plants alike.

14. I’ll use 8 plants in each treatment group.
A Step-by-Step Experiment
Experimental Design: Replicate
I’ll use 8 plants in each treatment group.

15. Experimental Design: Randomly Assign
A Step-by-Step Experiment
Experimental Design: Randomly Assign
To randomly divide the plants into three groups, first I’ll label the plants with numbers 00 – 23. I’ll look at pairs of digits across a random number table. The first 8 plants identified will go in Group 1, the next 8 in Group 2, and the remaining plants in Group 3.

16. Experimental Design: Make a Picture
A Step-by-Step Experiment
Experimental Design: Make a Picture
Group 1
8 Plants
Treatment 1
control
24 tomato plants from a garden store
Compare juiciness and tastiness
Group 2
8 Plants
Treatment 2
½ dose
Group 3
8 Plants
Treatment 3
Full dose

17. Specify any other details, including how to measure the response.
A Step-by-Step Experiment
Specify any other details, including how to measure the response.
I will grow the plants until the tomatoes are mature. I’ll harvest the tomatoes when ripe and store them for evaluation. I’ll set up a numerical scale of juiciness and one of tastiness for the taste testers.

18. A Step-by-Step Experiment
Display the Data
I will display the results with side-by-side boxplots to compare the three treatment groups. I will compare the means of the groups.

19. Decide of the results are meaningful.
A Step-by-Step Experiment
Decide of the results are meaningful.
If the differences in taste and juiciness among the groups are greater than I would expect by knowing the usual variation among tomatoes, I may be able to conclude that these differences can be attributed to treatment with the fertilizer.

20. Today’s Assignment:
READ CHAPTER 13! There is a lot of vocab in this chapter. Be sure to read and take notes from the book.