TM 5-6635-386-12&P
the counts will equal the square root of the
1-11. MANUAL EVALUATION
average count or mean.
Take the same series of ten numbers. Use 1/4 minute
Given
One standard deviation equals 68% of the
time key.
population of events for a series.
If the standard deviation of the series is to lie within
Put these all together and:
+
average and this is to include 68% of the series,
-
+
Averagecount = 68%Prob.
then this is merely stating that 68% of the numbers
+ 1 Std Dev =
-
+
should lie within
of the average or mean and that
-
This finding can be put to good use in evaluating tester
32% should be outside the average or mean.
performance if we use it to inspect a series of counts
taken in the standard count mode.
That is, out of a series of ten numbers, 32% or about 3
out of 10 will lie outside plus or minus the square root of
(The procedure may also be used to evaluate any series
the average.
of nuclear tester counts. It is only necessary that the
tester not be moved during the series of counts. Counts
We simply add up a series of numbers, average them,
could be taken in any operating position, with another
determine the square root of the average and then add
tester nearby, etc.).
and subtract this square root to the average.
Two evaluation methods will be described.
The resultant high and low limits will include 68% of the
numbers in the series and 32% will be higher or lower
One uses a hand calculator with statistic functions, the
than the limits.
other uses a manual checkoff method. Both techniques
work well. Both should be used with ten successive
The following example illustrates a typical tester placed
counts taken in the 1/4 minute time period. (This will not
in Standard Count configuration and using the Student
work if counts are taken in other time periods due to the
Field Data Worksheet (DA Form 5448R (Moisture and
averaging or normalizing of the tester on long time
Density Tester Field Data Worksheet)), Figure 1-3, to
periods).
accumulate the data for ten counts. DA Form 5448-R is
located at the back of this TM for local reproduction
1-10.
CALCULATOR
EVALUATION
USING
authority. This tester was normal in all respects.
STANDARD DEVIATION
We seldom are fortunate enough to have the series work
Enter the series of numbers into the calculator following
out exactly 3 out of 10 each time we run a series.
calculator instructions for standard deviation. Determine
However, if we make an intelligent allowance for
the deviation and place it in one of the calculator's
variations in numbers we will observe that the series will
memories.
exhibit a general trend towards 3 out of 10. An
occasional 2 out of 10 will be observed and an
Pull up the mean of the series of counts and determine
occasional 4 out of 10 will be observed. However, the
the square root of the mean.
statistic probability of a 5 out of 10 or a 1 out of 10 is very
slim, and such splits should be very rare for a normal
Divide the deviation by the square root.
tester.
From the above discussion, if the series was "perfect",
It is important to observe the average from one such
the square root would equal the deviation and the
Standard Count Evaluation to another. If the average is
division would be "1.0". The series will seldom work out
within approximately 1/3 to 112 of the square root of the
perfectly, however, and some variance from 1.0 will be
prior average, then the difference between the two series
observed. It is normal for this to lie between 0.75 and
is normal.
1.25 with a general tendency towards 1.0 for a normal
tester.
1-10
