Definitions: 1) Observational study - observe outcomes without imposing any treatment 2) Experiment - actively impose some treatment in order to observe the response
Hippity Hop Rabbit Food I’ve developed a new rabbit food, Hippity Hop. Makes fur soft & shiny! Increases energy! 100% of daily vitamins & essential oils!
Can I just make these claims? NO What must I do to make these claims? Do an experiment Who (what) should I test this on? Rabbits What do I test? The type of food
3) Population – the entire group of individuals I am interested in 4) Sample – a part of the population we can examine closely.
5) Variable of interest – what you measure 6) Treatment – a specific experimental condition applied to the subjects or units
Iplan to test my new rabbit food. What is my sample? What is my treatment? What is the variable of interest? The rabbits I choose for my experiment Type of food How well they grow
Hippity Hop I’ll use my pet rabbit, Lucky! Since Lucky’s coat is shinier & he has more energy, then Hippity Hop is a better rabbit food!
7) Control group – a group that is used to compare the factor against; can be a placebo or the “old” or current item 8) 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!
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 6 2 4 5 9 10 1 7 3 8 Number the rabbits from 1 – 10. The remaining rabbits get the old food. The first five numbers pulled from the hat will be the rabbits that get Hippity Hop food. Place the numbers in a hat. 5 8 7 3 9 I evaluated the rabbits & found that the rabbits eating Hippity Hop are better than the old food!
9) blinding - method used so that units do not know which treatment they are getting 10) double blind - neither the units nor the evaluator know which treatment a subject received
Hippity Hop 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 Design • Control methods - reduces extraneous variables on the response; reduces variation in replication • Replication - the ability to repeat trials or an 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. Plots of land used for experiment Sample? Treatment? Variable of interest? 3 Types of fertilizer Yield of crop
Example 2: 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? No, no treatment was imposed on people.
Example 3: 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? Yes, a treatment is imposed. What is the treatment? Variable of interest? Four different word-processing programs speed
Randomization reduces bias (so it reduces error) by spreading any uncontrolled confounding variables evenly throughout the treatment groups. Is there another way to reduce variability? Variability is controlled by sample size. Larger samples produce statistics with less variability.
High bias & high variability High bias & low variability Low bias & low variability Low bias & high variability