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,



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?