STAT 541: Randomization (How To)
Statistics 541: Randomization (How To)
- no homework this week (yea!)
As treated vs intent to treat
- doesn't it make more sense to look at those who get the drugs?
- Called "as treated" (obviously named by the evil empire!)
- Average treated - average untreated is hopelessly biased
- Suppose you have three types:
- always takers
- never takers
- proper subjects (as assigned takers)
- Then we can try to estimate the third group
Controlled experiments/random sampling
- Chase down some goats
- 100 mice are in a cage, pick 50 for treatment, leave remaining
50 for control
- Slow goats/mice are sampled!
- Must use randomization device
- JMP will generate random Bernulli trials
- Problem with Bernuilli trials is that they are independent.
- You might get everyone under treatment!
- Better is to use a multinomial (fancy word for pulling cards
from a deck without replacement)
- Blocking for treatment vs control, why not also block for male
- Called stratification.
- Good idea: but male/female, young/old, black/white,
healthy/sick. Now we have one person per cell.
- Talk to doctor to see what is likely to be important and
control for the single most important issue.
- What if you want to control for 10 things?
- You need at least 1024 subjects!
- If you use 11 subjects and regression you are fine. (Ok, 40 would be more realistic)
- So do a multiple regression on things you want to control for
- Just don't tell anyone! Call it "adjusted" or something else
- Note: Toxicity can't be used as a covariate. Why?
Some math on efficiency
- Controling ahead of time doesn't cost a degree of freedom
- Controling afterward costs a degree of freedom
- No bigee-if you have at least 50 observations. Bigee if you have only 10 observations.
Last modified: Mon Apr 23 17:38:57 2001