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Topic: Stratified versus Block
Replies: 0

 Bob Hayden Posts: 2,384 Registered: 12/6/04
Stratified versus Block
Posted: Nov 14, 1996 9:39 PM

----- Forwarded message from AlCoons@aol.com -----

We are trying to distinguish between Stratified Random Sampling and Block
Design. BPS makes the point:

"Blocks and strata both group similar units. We use two different names
only becuase the idea developed separately for sampling and experiments."
pg 214

So what is the party line? Are they the same? Is there a difference?

Al

----- End of forwarded message from AlCoons@aol.com -----

I'd say it is the same concept applied to different situations.

Here's another example. When I teach linear models I present analysis
of covariance in the following way.

Sometimes we want to study the effect of one or more variables x1,
x2, ... on another variable, y. We ALREADY know that y also depends
on z1, z2, ..., and if we do not include the z's in the model, we will
get a very poor model. So we include the z's to get a good model, and
then test hypotheses about the x's.

Or as Joe Ward would say, "If you can't control for it, measure it."

In the traditional situation, the x's represent cells in an ANOVA and
there is one z and it is a measurement variable. So, is what I do
REALLY analysis of covariance? I'd say it is the same idea applied to
teaching, and see no reason to say much about the narrower idea.
However, if you were going to do a literature search, it might be
useful to know that virtually all of the literature you turned up
would be about the narrower situation.

Similarly, if you are teaching blocking and stratified sampling, I
don't think it is important to draw a distinction. However, if you
want to do a literature search it would be useful to know that the
history of this basic idea within experimental design has been
different than its history within survey sampling, so you might need
to search both areas, using the locally accepted terminology in each,
if you wanted to research the impact of this basic idea in
statistics.

But for purposes of AP Stats., I think what David Moore says is
exactly what should be said on this topic (indeed, on MOST topics!).

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