Can someone define stratified sampling for me?

Question

**READ THIS** Please don't just copy it off a website.. I need a really simple definition so I can understand it. Thank you

Answer 1

I'm thinking what you mean is this: If you have to take rock samples (or whatever) from a 100 meter high rock wall, it isn't typically possible to sample every centimeter of the section (time, money, weight). Instead, you subsample every few meters (spacing depends on the purpose of your study); e.g. take one kilogram samples every five meters for a total of 20.

If there are interesting spots within a specific subsection (e.g. most of the wall is black rock but there is a portion of white rock cutting through it), you might take an extra few samples there; the black rock below, the white rock, and the black rock above.

It is important to take pictures, really good field notes, and to label all your specimens.

Answer 2

Taking a sample from each layer of strata (bedding) so as to get a non-biast sample.  Different layers contain different properties so you should take a chip off them all to get coherent sample

Answer 3

"Stratified" usually means "layered", so in the context of earth sciences & geology this would usually imply sampling of layered sediments or sedimentary rocks.

As a geologist, you might, for example, sample the rock samples, rock types or fossils working up a geological sucesion, for example, a cliff face, as opposed to walking along the section in which case you would usually observe an entirely different variation.

You might wish to examine soft sediments by taking shallow boreholes or auger (core) samples and focussing on the different layers (strata) as you work downwards into deeper and older sediment.

Other things than rocks and sediments can be stratified; for example, water bodies (lakes, seas), which can be layered according to temperature, acidity, oxygen content, suspended sediment content and microfloral content.

Other than that, this is difficult to answer because the question is pretty vague...