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Ah, complete enumeration sampling is like painting all the happy little trees in a forest. It's when you gather data from every single member of a population, leaving no one out. Just imagine each data point as a unique color on your palette, coming together to create a beautiful and accurate picture of the whole population.
square method
Factor trees and factor rainbows.
As the wikipedia article on this subject suggests, systematic sampling is most readily applied when potential sample elements are linearly ordered either in time or space. For example, one could choose to include every fifth customer arriving at a store in one's sample, which would be an instance where sample elements are ordered in time. The difficulty with many research situations in biology is obviously that sample elements are not linearly ordered. A herd of buffalo in a grassy field, for example, or a collection of microorganisms on a microscope slide. Remedies depend on circumstances. Suppose you want to apply systematic sampling in a small forest where you want to estimate the fraction of trees infested with a certain species of insect. You decide on, say, a one in five sample and that you will include 500 trees in your sampling frame in order to get a sample size of 100. To begin you walk enough parallel transects through the forest, marking sufficiently large trees as you go, to get your 500-tree sampling frame. Then you take a second trip through along transects to identify infested trees.
The Vietnamese paint their trees with a white insecticide at the bottom of the trees to kill pest before they can damage crops. Wrapping the trunk in burlap is also another method of controlling the pests.