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APPENDIX - Statistical analysis
General definition
BATCH
A set of similar measurements that are to be subjected to statistical analysis.
CHARACTERISTIC TYPE
Bilateral characteristic: this type of characteristic has no natural limits, therefore it is
necessary to indicate specification limits; e.g. a diameter that may have a dimension
greater or less than the design.
Single limit characteristic: this type of characteristic has a single limit indicating an
acceptability threshold, so only one specification limit need be indicated; e.g. a
breaking strain control, where the part is considered acceptable if the force required
to break it exceeds a certain level. This differs from a characteristic with a single,
upper limit if the acceptable threshold represents the maximum value that the
characteristic may assume.
Unilateral characteristic: this type of characteristic has a natural, “Insuperable” limit;
e.g. a parallelism that may not fall below 0. In this case, the specific limit is indicated,
as well as the physical limit, if other than zero.
TOLERANCE LIMITS
Two tolerance limits can be defined for a characteristic:
UTL - Upper Tolerance Limit: represents the upper specification limit
LTL - Lower Tolerance Limit: represents the lower specification limit.
TOLERANCE INTERVAL
Also know simply as tolerance, it represents the acceptable range of measurement
values for a given characteristic. It is determined using different methods, depending
on the characteristic type:
Bilateral characteristic: UTL - LTL
Unilateral characteristic: calculated as |TL - PL|, where TL is the tolerance limit and
PL is the physical limit (usually 0).
Single limit characteristic: cannot be calculated.
TOLERANCE MID-POINT
The mid-point of the tolerance interval.
It is determined using different methods, depending on the characteristic type:
Bilateral characteristic: (UTL – LTL) / 2
Unilateral characteristic: calculated as |TL - PL| / 2.
Single limit characteristic: cannot be calculated.
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