What is the difference between stratifying and blocking?

Answer 1

Pretend you have a mountain of dirty clothes. You want to see the difference between washing dark and light colored clothing with warm and cold water. You need a sample because washing all of them takes too long.

Stratifying means you separate the clothes into dark and light colors and you pick a simple random sample from EACH color pile (this ensures you have the same number of clothes for both colors). Then you combine them into a miniature sample and you use RANDOM ASSIGNMENT to assign the clothes in the sample to either cold or warm water treatment.

Blocking also means that you separate the clothes into dark and light colors and you pick a simple random sample from each color pile. However, instead of combining it to make a miniature sample, you keep them separate and use random assignment WITHIN these blocks. Eg. for the dark color pile, you use random assignment to assign half to cold, half to warm water washing, and you repeat the same process for the light color pile, keeping them separate the entire time.