1. Observational Studies and Experiments
Ch. 10
Observational Studies and Experiments

2. Definitions:
1) Observational study - observe outcomes without imposing any treatment
Retrospective studies collect data from the past
Prospective studies identify subjects in advance and collect data as the events unfold
2) Experiment - actively impose some treatment in order to observe the response

3. 100% of daily vitamins & essential oils!
I’ve developed a new rabbit food, Hippity Hop.
Makes fur soft & shiny!
Hippity Hop
Rabbit Food
Increases energy!
100% of daily vitamins & essential oils!

4. Can I just make these claims?NO
What must I do to make these claims?
Do an experiment to prove it
Who (what) should I test this on?
Rabbits
What do I test?
The type of food

5. 3)Experimental unit – the single individual (person, animal, plant, etc.) to which the different treatments are assigned
4) Factor – is the explanatory variable
5) Level – a specific value for the factor

6. 6) Response variable – what you measure
7) Treatment – a specific experimental condition applied to the units

7. I plan to test my new rabbit food. What are my experimental units?
What is my factor?
What is the response variable?
Rabbits
Type of food
How well they grow

8. 8) Control group – a group that is used to compare the factor against; can be a placebo or the “old” or current item
9) Placebo – a “dummy” treatment that can have no physical effect

9. 10) blinding - method used so that units do not know which treatment they are getting
11) double blind - neither the units nor the evaluator know which treatment a subject received

10. 12) Confounding variable : the effect of the confounding variable on the response cannot be separated from the effects of the explanatory variable (factor)

11. (REQUIRED!) Principles of Experimental Design
Control of effects of extraneous variables on the response – by comparing treatment groups to a control group (placebo or “old”)
Replication of the experiment on many subjects to quantify the natural variation in the experiment
Randomization – the use of chance to assign subjects to treatments

12. The ONLY way to show cause & effect is with a well-designed, well-controlled experiment!!

13. I’ll use my pet rabbit, Lucky!
Hippity Hop
Since Lucky’s coat is shinier & he has more energy, then Hippity Hop is a better rabbit food!
Discuss the need for a comparison group.

14. Old Food
Hippity Hop
Now I’ll use Lucky & my friend’s rabbit, Flash. Lucky gets Hippity Hop food & Flash gets the old rabbit food.
WOW! Lucky is bigger & shinier so Hippity Hop is better!
Discuss the need for replication.

15. The Hippity Hop rabbits have scored higher so it’s the better food!
Old Food
Hippity Hop
The first five rabbits that I catch will get Hippity Hop food and the remaining five will get the old food.
Discuss the need for random assignment to treatment groups.

16. Place the numbers in a hat.
Number the rabbits from 1 – 10.
Old Food
Hippity Hop
The remaining rabbits get the old food.
The first five numbers pulled from the hat will be the rabbits that get Hippity Hop food. 6 1 2 10 4 5 7 9 8 3 8 5 7 3 9 4
I evaluated the rabbits & found that the rabbits eating Hippity Hop are better than the old food! 1 5 7
Discuss the need for the evaluator to be blinded 10 3 2 6 8 9

17. Completely Random Design
Group 1
5 Rabbits
Treatment 1
Hippity Hop Food
Compare
Growth
Random Allocation*
10 Rabbits
Treatment 2
Placebo
Group 2
5 Rabbits
* Assign numbers to rabbits (01-10). Write each number on a slip of paper. Put all papers (make sure they are the same size) in a hat, mix up the slips and then draw numbers. The first 5 are assigned to the treatment group. The remainder were placed into the placebo group.

18. Hippity Hop Rabbit Food makes fur soft and shiny, & increases energy for ALL types of rabbits!
Discuss scope of inference.
Can I make this claim?

19. Experimental units? Factors? Levels? Response variable?
Example: A farm-product manufacturer wants to determine if the yield of a crop is different when the soil is treated with three different types of fertilizers. Fifteen similar plots of land are planted with the same type of seed but are fertilized differently. At the end of the growing season, the mean yield from the sample plots is compared.
Experimental units?
Factors?
Levels?
Response variable?
How many treatments?
Plots of land
Type of fertilizer
Fertilizer types A, B, & C
Yield of crop 3