Exactly! No one should care about the mean: they should care about what
part of the distribution they want protection for and for what part of
the distribution they'll take the risk. This boils down to a
conceptually simple economic cost/loss ratio: what's the cost of
protection vs the cost incurred when protection fails?
Kim N5OP
On 8/9/2015 11:19 AM, Jim Lux wrote:
On 8/8/15 6:11 PM, Roger (K8RI) on TT wrote:
I'm glad Kim posted that information, but I wish the scientific
community would go back to averages rather than median. The average is
much more meaningful than knowing the number that lies half way between
the highest and lowest figure measured, at least they are for me. Median
is an interesting number, but average seems to be much more informative
and typically what is used for design. With a number of samples large
enough to be statistically valid a single, significant outlier, be it
high or low can substantially skew the median, but have little effect on
the average.
Actually, the median is the one you want: half the strokes are higher,
half the strokes are lower. it is not skewed as much by a outlier.
The mean (arithmetic sum divided by the number of entries) would be
skewed by a single very large number.
In practice, what you'd really want is something like the 95th or 99th
percentile. And that's something that's in the standard scientific
literature. I think pretty much any of Uman's books has a figure in
it of stroke current distribution.
http://www.iclp-centre.org/pdf/Invited-Lecture-3.pdf
has a lot of interesting charts..
On page 10, it shows that 50% of the first strokes are around 35 kA or
less, and 90% are <100kA peak.
http://surgelogic.com/documents/technotes/Lightning_Stroke_Tech_Note_1300DB1101.pdf
says 95% of positive strokes <30kA, 98%<60kA
for negative strokes, 82%<30kZ, 98%<60kA
The so called "super strikes", or Positive lightening, which is
associated with sprites can move the median, but happen so seldom, they
have little effect on the average and when it come to lightening, do we
design for the median, average, maximum, or the best we can afford?
Design for what you can afford to lose. I suspect that designing for
the 99th percentile is inappropriate for hams..
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--
Kim Elmore, Ph.D. (Adj. Assoc. Prof., OU School of Meteorology, CCM, PP
SEL/MEL/Glider, N5OP, 2nd Class Radiotelegraph, GROL)
/"In theory there is no difference between theory and practice. But, in
practice, there is." //– Attributed to many people; it’s so true that it
doesn’t matter who said it./
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