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re: pH measurement (accuracy or precision?)

Just because something is accurate, does not mean it is precise!
I say that the most
misleading instrument is one that is accurate but not
precise(provided you have a way of
calibrating your readings).  You really need both accuracy and
precision.  Liken it to
shooting a gun(bad example this week, but easiest for me to
explain).  If you shoot 5
shots and they are all within an inch of each other, but 6 inches
away from the bullseye,
then it is precise, but not accurate.  That can be compensated
for.  If, however, you
shoot 5 shots and there are some that are 3 inches away from the
bull, some dead center
and one that is 3 inches away in the opposite direction, then it
is more accurate(each is
closer to the target), but less precise than your first 5 shots.
Some of the shots are 6
inches away from each other!   There is no way to compensate for
this.  When you
translate this back to test equipment, you see that precision is
really more important,
but accuracy(or some way to calibrate and zeroize your equipment)
is also vital.  I've
never had a pH meter, I don't know how they work, but if they can
be adjusted at all, or
if they have a way of calibrating them(as I've heard), then I
would go for the more
precise instrument because you can always compensate for
consistent inaccurate

> Date: Fri, 23 Apr 1999 10:19:38 -0500
> From: George Slusarczuk <yurko at warwick_net>
> Subject: Re: pH measurement (was APD V3 #986
> Hello James,
> What you should be after is greater *accuracy* in pH
measurements. The
> precision will follow. Using the common definition,
> Accuracy = how close is your reading to the "true" value,
> Precision = how close to each other are the readings,
> it follows that if you have good accuracy, i.e. your reading is
> close to the true value, then, of necessity, the individual
readings can
> not be too far apart. The most misleading instrument is one
that is
> precise, but NOT accurate!
> <snip>
> Best,
> George

-Dave (in San Diego)
I'm sure you can figure out my e-mail address.