Reducing sampling error: Blocking
1)Organizing study by major components of phsyical locations / natural groupings
2)AND including this informationin statistical model!
3)within each block, study units receive each experimental treatment
4)Similar to “stratifying”
5)Easiest forms:
a) pairing
b) repeated measures (RM)
Blocking: common forms
-Humans: class, school, school distriction hospital
eg, study effect of drug, use 4 hospitals as blocks
w/in each hospital randomly assigning drug treatment & placebo
what would be wrong with using placebo just at 2 hospitals & expt. drug at 2 others?
Blocking in ecology
-Ecology: eggs in nest,
-wildlife managment unit,
-lakes,
-stream
Often multiple treatments (3+) w/in blocks
-Fertilizer experiment:
-control, N, P, K treatments w/in the same block
Issue w/blocking:
-makes stats tougher
-Usually good to have 4+ blocks
Ex: Blocking by nest to determine impacts of avian malaria
- 1/2 of nestlings get anti-malarial
- 1/1 get saline injection
Blocking of Trillium Trail Deer Exclosures
-What is advantegous about blocking for forest ecology experiments?
-Why would a completley random plot location have had drawbacks?
Blocking’s easy forms: pairing & repeated measures (RM)
Example: A natural experiment using pairing for its controls
Example: A natural experiment using pairing for its controls
Example: A **natural experiment*8 using pairing for its controls
Example: A natural experiment using pairing for its controls
- What does this design accomplish?
- Would this be necessary if you were studying corn in a plowed field?
- How might this be converted to a true experiment?
Repeated measures (RM)
-similar to blocking / pairing
-measurements conducted before vs. after treatment delivered
-individuals serve as own controls
-common in human studies
-example:
-heart rate b/f vs. after drinking Red Bull
-WBC b/f vs after chemo therapty
-Often/Ideally will still have un-treated controls in case any change occurs over time due to environmental conditiosn
-unlike blocking, stats for simple pairing / RM are usually easy
-but, need to have enough replicates for sufficient “power”
Blocking - Book’s definition:
“blocking is the grouping of experimental units that have similar propoerties. Within each block, treatments are randomly assigned to experimental units.”
Difficulties w/ blocking:
-Contamination of nearby treatments (“spillage”“) # -Edge effects # -Example: ## -In my fenced deer exclusion plots, roots grow out of the plot into the control and compete with plants still being eaten
