Blocking
Blocking divides experimental units into homogeneous groups (blocks) based on some characteristic that is known to affect the response variable. Treatments are then randomly assigned within each block. This reduces experimental error by controlling for known sources of variation while maintaining randomisation.
Real World
An agricultural researcher at Rothamsted Research testing three fertilisers blocks the trial by field slope — flat, moderate, steep — so that drainage differences don't mask fertiliser effects.
Exam Focus
Always identify the blocking variable and explain why it is expected to affect the response before describing randomisation within blocks.
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