chapter 2 experimental design. definitions: 1) observational study - observe outcomes without...
TRANSCRIPT
Chapter 2Chapter 2
Experimental Design
Definitions:1) Observational study -
observe outcomes without imposing any treatment
2) Experiment - actively impose some treatment in order to observe the response
I’ve developed a new rabbit food, Hippity Hop.
Rabbit Food
Makes fur soft & shiny!
Increases energy!100% of
daily vitamins & essential
oils!
Can I just make these claims?
What must I do to make these claims?
Who (what) should I test this on?
What do I test?
NO
Do an experiment
Rabbits
The type of food
3)Experimental unit – the single individual (person, animal, plant, etc.) to which the different treatments are assigned
4) Factor – is the explanatory variable5) Level – a specific value for the factor
6) Response variable – what you measure
7) Treatment – a specific experimental condition applied to the units
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
Hippity HopI’ll use my pet rabbit, Lucky!
Since Lucky’s coat is shinier & he has more energy, then Hippity Hop is a better rabbit food!
8) Control group – a group that is used to compare the factor against; can be a placebo or the “old” or current item9) Placebo – a “dummy” treatment that can have no physical effect
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!
WOW! Lucky is bigger & shinier so Hippity Hop is better!
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.The Hippity Hop rabbits
have scored higher so it’s the better food!
Old Food Hippity Hop
Number the rabbits from 1 – 10.
Place the numbers in a hat.
1
2
3
45
6
78
9 10
The first five numbers pulled from the hat will be the rabbits that get
Hippity Hop food.
I evaluated the rabbits & found that the rabbits eating Hippity Hop are better than the old food!
The remaining rabbits get the old food.
39857
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
Rabbit Food
Hippity Hop Rabbit Food makes fur soft and shiny, &
increases energy for
ALL types of rabbits!Can I
make this
claim?
Principles of Experimental Principles of Experimental DesignDesign
• 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
The ONLY way to show cause & effect is with a well-designed, well-controlled experiment!
The ONLY way to show cause & effect is with a well-designed, well-controlled experiment!!The ONLY way to show
cause & effect is with a well-designed, well-controlled experiment!!!
Example 1: 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, & CYield of crop
3
Example 2: A consumer group wants to test cake pans to see which works the best (bakes evenly). It will test aluminum, glass, and plastic pans in both gas and electric ovens.
Experiment units?
Factors?
Levels?
Response variable?
Number of treatments?
Two factors - type of pan & type of oven
Type of pan has 3 levels (aluminum, glass, & plastic & type of oven has 2 levels (electric & gas)
How evenly the cake bakes
6
Cake batter
Example 3: 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.Why is the same type of seed used on all 15 plots?What are other potential extraneous variables?Does this experiment have a placebo? Explain
It is part of the controls in the experiment.
Type of soil, amount of water, etc.
NO – a placebo is not needed in this experiment
Experiment Experiment DesignsDesigns
•Completely randomized – all experimental units are allocated at random among all treatments
Treatment group 1
Treatment group 2
Treatment group 3
explanatory
variable
response
variable
Ra n
dom
ass
ignm
e nt
Treatment C
Treatment B
Completely randomized design
Randomly assign experimental units
to treatments
Treatment A
Treatment D
•Randomized block – units are blocked into groups and then randomly assigned to treatments
Ran
dom
ass
ignm
entGroup1
Group2
Treatment 1Treatment 2Treatment 3
Treatment 3Treatment 2Treatment 1
explanatory response
varaible varaible
Treatment B
Randomized block design
Randomly assign experimental units
to treatments
Treatment A
Put into homogeneous groups
Treatment A Treatment B
– match up experimental units according to similar characteristics & randomly assign on to one treatment & the other automatically gets the 2nd treatment
– have each unit do both treatments in random order
– the assignment of treatments is dependent
•Matched pairs - a Matched pairs - a special type of block special type of block designdesign
Pair experimental units according to
specific characteristics.
Next, randomly assign one unit from a pair to Treatment A. The other unit gets Treatment B.
Treatment A Treatment B
This is one way to do a matched pairs design – another way is to have the individual unit do both treatments (as in a taste test).
12) Confounding variable – the effect of the confounding variable on the response cannot be separated from the effects of the explanatory variable (factor)
Suppose we wish to test a new deodorant against one currently on the market.
Treatment B
Treatment & group are confounded
Treatment A
Treatment A Treatment B
One group is assigned to
treatment A & the other group to treatment B.
ConfoundingConfounding does NOT occur in a
completely randomized design!
Example 4: An article from USA Today reports the number of victims of violent crimes per 1000 people. 51 victims have never been married, 42 are divorced or separated, 13 are married, and 8 are widowed.Is this an experiment? Why or why not?
What is a potential confounding variable?
Age – younger people are more at risk to be victims of violent crimes
No, no treatment was imposed on people.
Example 5: Four new word-processing programs are to be compared by measuring the speed with which standard tasks can be completed. One hundred volunteers are randomly assigned to one of the four programs and their speeds are measured.
Is this an experiment? Why or why not?What type of design is this?
Factors? Levels?
Response variable?
Yes, a treatment is imposed.
Completely randomizedone factor: word-processing program with 4 levels speed
Example 5: Four new word-processing programs are to be compared by measuring the speed with which standard tasks can be completed. One hundred volunteers are randomly designed to one of the four programs and their speeds are measured.
Is there a potential confounding variable?
Can this design be improved?
Explain.
NO, completely randomized designs have no confounding
You could do a block design where each person uses each
program in random order.
Example 6: Suppose that the manufacturer wants to test a new fertilizer against the current one on the market. Ten 2-acre plots of land scattered throughout the county are used. Each plot is subdivided into two subplots, one of which is treated with the current fertilizer, and the other with the new fertilizer. Wheat is planted and the crop yields are measured.What type of design is this? Why use this method?
When does randomization occur?
Matched - pairs design
Randomly assigned treatment to first acre of each two-acre plot
Randomization reduces bias by spreading any uncontrolled confounding variables evenly throughout the treatment groups.
Variability is controlled by sample size. Larger samples produce statistics with less variability.
Blocking also helps reduce variability.
Is there another way to reduce
variability?
High bias & low variability
Low bias & low variability
Low bias & high variability
High bias & high variability