InstroTek
61
Expressing the
x
value in them of the
value plus a factor of
the deviation:
n
k
Z
k
x
From the Normal tables, for 95% confidence, the Z value is 1.96.
96
.
1
1
196
.
n
Z
K
Thus the new reading should be equal to the average of the
old reading plus/minus 1.96 times the square-root of the old
average.
This is true for the 60-second count which is direct. For another
time base, the K term must be reduced by the square-root of
the count pre-scaling (e.g. for a 240-second count which is 4
times as long as the direct 60-second count, the new reading
should be plus/minus 0.98):
98
.
0
4
96
.
1
n
Z
K
This is the case when the standard count is taken which involves
240 each (n=240/60=4) 1-second counts. A new standard count
should be equal to the old standard count plus/minus 0.98 times
the square-root of the old standard count 95 percent of the
time.
EXAMPLE:
The average of the daily standard count for the last month is
10,000. The square-root of this average is 100. A new standard
count (240 each at 1 seconds, but displayed as 60 seconds,
CPM) should be between 9,902 and 10,098 with a 95% of
probability.
