84
Statistical Calculations
6.6
Statistical Calculations
The instrument continually calculates statistics for a maximum of 1,000,000 measurement data
points, and the results of those calculations can be displayed on the Measurement screen. (p. 85)
In addition, the results can be printed. (p. 143)
Statistical calculation stops once the number of data points reaches 1,000,000. It can be resumed
by clearing the statistical calculation results.
Definition of statistical calculations
The instrument calculates the average value, maximum value, difference between the maximum
and minimum values, minimum value, population standard deviation, sample standard deviation,
and process capability index.
Maximum value
Minimum value
Maximum value - Minimum value
X
max -
X
min
Average value
Population standard deviation
Sample standard deviation
Process capability index* (variability)
Cp
UPP LOW
n
=
−
−
|
|
6
1
σ
Process capability index* (bias)
Cpk
UPP LOW
UPP LOW
x
n
=
−
−
+
−
−
|
| |
|
2
6
1
σ
* The process capability index expresses the ability of the process to achieve the target quality in terms of its
quality variability and bias width. In general, process capability can be evaluated as indicated below based
on the
Cp
and
Cpk
values:
Value
Process capability
Cp
and
Cpk
> 1.33
Adequate
1.33 ≥
Cp
and
Cpk
> 1.00
Appropriate
1.00 ≥
Cp
and
Cpk
Inadequate
•“UPP” and “LOW” refer to the comparator’s upper limit and lower limit values.
•When the comparator function is set to
OFF
, the process capability index is not calculated.
•When the number of valid data points is 1, the sample standard deviation and process capability
index will be displayed as 0.
•When
σ
n−1
, is 0,
Cp
and
Cpk
will be 99.99.
•The upper limit for
Cp
and
Cpk
is 99.99. If either value is greater than 99.99, it will be displayed
as 99.99.
•When
Cpk
is negative, it will be treated as 0.
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