1: An AP stats test asked, "What is the primary benefit of blocking?"
For me, it depends on the goal.
A. In statistical inference (in estimating the value of a mean or
in determining the significance of a difference in means)
I agree that A primary benefit is "reducing variance/variation".
But A primary benefit is obtaining a tighter confidence interval or
increasing the statistical significance of an observed
difference.
To say that one is more primary than the other is ambitious.
If forced to choose, I'd say that "reducing variation" comes in
second
as a means to the ends of statistical inference.
B. In interpreting the results of a study, the primary benefit of blocking
(assuming the association is unlikely if due to chance),
is to "better measure the direct effects of a known cause" (experiment)
or "to strengthen an argument for direct causation" (observational
study*).
Eliminating a confounding factor (eliminating an alternate explanation)
is simply a means to these ends.
* This assumes that "blocking: is applicable to observational studies
(see following point),
#2: Question: Is the term 'blocking' limited only to experiments where one
has 'control of' the [assigned] values of a variable -- or is it
applicable to
observational studies where one measures (controls for) the existing
values of variables?
Joe Ward wrote in message <001b01bf325f$08824420$14880e3f@waradthinkpad>...
>Steve --
>
>Your students are asking the good questions!! This comes up
>repeatedly.
<snip/snip/snip>
>-- Joe
>*************************************************************
>Joe Ward Health Careers High School
>167 East Arrowhead Dr 4646 Hamilton Wolfe
>San Antonio, TX 78228-2402 San Antonio, TX 78229
>Phone: 210-433-6575 Phone: 210-617-5400
>Fax: 210-433-2828 Fax: 210-617-5423
>[EMAIL PROTECTED]
>http://www.ijoa.org/joeward/wardindex.html
>*************************************************************
>----- Original Message -----
>From: SUGHRUE, STEVE <[EMAIL PROTECTED]>
>To: <[EMAIL PROTECTED]>
>Sent: Thursday, November 18, 1999 7:15 PM
>Subject: blocking for variation/confounding
>
>
>| Hi everyone!
>| In the AP course description booklet, multiple choice question
>| number 13 asks for the primary reason for blocking when designing an
>| experiment. My students and I agree that reducing variation is a good
>| answer, but isn't reducing confounding also pretty good? Are we missing
>| something here??
>| Thanks to anyone who can help .....
>|
>| Steve Sughrue
>| Tabor Academy
>| Marion, MA
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