Posted for "Jim Nordin" <panelmaker@earthlink.net>:

 Gary did a pretty good job in my view. And I'd like to further the
 discussion with reference to accuracy.
 
First I want to tell you all I have no flame suit. After seeing recent
 messages on various subjects, I hesitate to make comments on the LML,
 perhaps others do as well. This is bad. But I'm going to make these and see
 how it goes.
 
 There is a perception, by many, that accuracy and precision is one in the
 same. While precision is not a topic of this discussion per se it might be
 good for everyone to have an appreciation for both, thus bringing all of us
 to a level understanding of both and we will all be better to express our
 views and ideas. Recently I read an article regarding accuracy and precision
 as it refers to GPS. Which also might be worthwhile having an appreciation
 for. [I've been trying to educate friends on the "accuracy" information on
 GPS packaging and literature for a long time.]
 
 Accuracy is a measure of the closeness of one or more "rate of burn"
 readings to a "rate of burn" that's known and defined in terms of an
 absolute reference system (how many gallons were left perhaps). The known
 reference often is referred to as the "true reference point" or in this case
 "rate of burn". Accuracy compares different measuring devices to a true
 "rate of burn", and hence to one another. [I guess to do this properly, one
 would have to get to altitude on one tank, switch to another tank, take the
 rate of burn readings onboard during steady state flight, switch to another
 tank after a measured period of time, land and determing delta gallons in
 the tank used during the reading period. Compare the rate of burn indicated
 on board with the usage over time as measured by fuel remaining.]
 
 Precision is a measure of repeatability indicating how closely several
 "rates of burn" readings fall within each other. Precision is based on a
 relative reference system that's unique to the device making the
 measurements.
 
 You can give the analogy:
 
 When dealing with target practice with a bow and arrow.
 
 If there is a tight grouping of arrows on the target in the upper left
 quadrant near the edge of the target, that grouping is said to be a precise
 grouping. Poor accuracy perhaps.
 
 Notice the tight grouping missed the bull's eye significantly. If a looser
 grouping were found around the bulls eye and the convergence was near the
 center of the bull's eye that would be said to be accurate. [Right on
 target?] Poor precision perhaps. If you were trying to hit a water filled
 balloon to deflate that balloon, you would want an accurate precise hit. For
 hitting all around the balloon might be categorized as accurate, you still
 didn't burst it.
 
 If a tight grouping of arrows were found in the bull's eye, that would be
 said to be an accurate precise grouping.
 
Fly safe by establishing all limitations, accurately and precisely.
 
 Jim