From: A.J. Rossini (rossini@blindglobe.net)
Date: Thu Jul 10 2003 - 20:19:12 EST
"Bryant, Jenny (NSW)" <JBryant@arcbs.redcross.org.au> writes:
>> I have the following advice to determine whether or not to average all
>> three readings in a set of 3 measurements:-
>>
>> Average the three Mean Channel values. To determine whether a value is
>> too low or too high to be averaged with the others use the formula:
>> Y = (X' - X")/(Xh - Xl)
>> Where (X' - X") is the difference between the 2 closest values, and (Xh -
>> Xl) is the difference between the highest and lowest values.
>> If Y is less than or equal to 0.25, average the closest values.
>> If Y is greater than 0.25, average all 3 values.
>>
>> We have been looking at a number of statistical methods and there appears
>> to be some truth in the formula however if 2 readings are the same the
>> third will always be excluded no matter how close it is. Also, we can't
>> figure out the derivation of the value 0.25.
>>
> Does anyone have any suggestions?
I've never seen that ad-hoc approach before, but here's my
interpretation with respect to 3 observations:
one of the closest two MUST be an endpoint (max or min, i.e. one
of X' and X" must be one of Xh or Xl).
therefore, if the third (not X' or X"; must be one of Xl or Xh) is
greater than 3 times the distance of the closest two (X'-X") from
the middle one, it is excluded.
3 times is maybe a reasonable cut off. I don't think that there is
any good justification with just 3 points (though the three points in
this case sound like summary measures, so there is some
accuracy/information behind them), but it doesn't sound like a bad
approach, as long as it is documented.
best,
-tony
--
A.J. Rossini / rossini@u.washington.edu / rossini@scharp.org
http://software.biostat.washington.edu/ UNTIL IT MOVES IN JULY.
Biomedical and Health Informatics, University of Washington
Biostatistics, HVTN/SCHARP, Fred Hutchinson Cancer Research Center.
FHCRC: 206-667-7025 (fax=4812)|Voicemail is pretty sketchy/use Email
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