Articles - Leather Technology Magazine Nº74
Application of the hypothesis analysis method using Cohen’s Kappa index to measure
the agreement between leather sorters
Authors: Dr. Patricia Casey1,*, Eng. Gustavo Altobelli1, Tech. Pablo
Pignatelli1
1,Fonseca Tannery (Curtiembres Fonseca S.A.), Buenos
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Application of the hypothesis analysis method using Cohen’s Kappa index to measure
the agreement between leather sorters
Key words: automotive leather; grading; surface defects;
sorting criteria; Cohen’s Kappa index.
Work presented at the XXX IULTCS Congress, Beijing, China, 2009
ABSTRACT
One of the more sensitive activities carried out in
a tannery, because of the impact the results could cause on
the customer and on the tannery's business is, certainly, the
sorting or grading of the hides in the different stages (wet
blue or wet white, crust and finished). The surface defects
affect the aesthetics appearance of leather and leather
goods as well as the usable area. If the grade assigned to a
hide by a sorter is lower than the one it really is, according
to the agreement with the customer it will cause economical
damage to the tannery. On the other hand, if the grade
assigned is higher than the one it really is, the customer
will be disappointed and claim. The detection of defects on
the surface of natural materials such as leather is a difficult
task because of the variety of shapes and textures, as well
as in the quality and quantity of defective areas. The most
common method applied in the leather industry to sort
or grade hides is visual inspection. Notwithstanding the sorting or grading criteria could be well defined and agreed
between the tannery and the customer, we can always wonder:
if we change from one sorter to another, which will be the
concordance level existing between them? In other words, how
far the sorters will coincide on their evaluation? Will the result
be a real or random one? Due to the variability between two or
more sorters which is traditionally recognized as an important
source of mistakes, from our point of view, it would be very
useful to transform an attribute analysis into a variable one.
The tanneries that supply leather to the automotive companies
have to certify their Quality System according to QS 9000
or ISO/TS 16949. One of the requirements in that norm and
technical specification is to apply the Automotive Industry
Action Group (AIAG) reference Manual "Measurement Systems
Analysis" which describes the study of the measurement
systems by attributes. In this paper, we describe the practical
way on which the agreement between sorters is measured
and the application of the hypothesis analysis method, cross
tabulation, calculating Kappa coefficient initially proposed by
Cohen, which is a statistical concordance rating measure for
qualitative (categorical items) between two sorters.
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