Exercise Quality Management - wzl.rwth-aachen.de · Time until pizza is delivered: Question: Less than 30 min.? Answer: That's normal. Question: More than 30 min.? ... Quality Function
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Today the House of Quality is one of the most commonly used tools for implementing Quality Function Deployment, providing a combination of various matrices, lists and tables, to support the individual transformation steps in QFD process. The conversion of Customer Requirements into Quality Characteristics is supported and evaluated by using weighted relations to link the contents of the various matrices and tables.
At first the QFD weightings are determined with a pair wise comparison of all requirements which have resulted from the survey. In the following all Customer Requirements in the matrix are compared with each other.
If a Customer Requirement (e.g. “A” from line 1) is more important than the other requirement in the pair (“B” from column 2), a “2” has to be inscribed in the appropriate matrix array. In case of the same importance a “1”, in case of less importance a “0” has to be inscribed.
To get the weighted Customer Requirements use the following formulas:
for a range of 1 to 5
for a range of 1 to 10
Wi : weight; Si : sum of correlating line; Smax : maximum line sum;
Smin : minimum line sum
Last but not least the weighted Customer Requirements are inscribed into the House of Quality.
Case study “car tyre”: Completely elaborated House of Quality
The picture shows the QFD on the production level by the example of a car tyre. The proceeding differs from the ASI concept and shows that the QFD works as a flexible instrument.
In the first House of Quality (HoQ) of this example it is not possible to name target values because the components can’t be defined in physical values. Therefore it is not possible to identify the direction of improvement and to sign in the matrix of the rooftop.
This HoQ analyses the influence of Customer Requirements on the product components.
Example – Planning of Components
The HoQ of the car tyre shows that the technical parameter “tread pattern” is related to most of the Customer Requirements. Now this parameter will be shown in detail.
The detailed view could also have been achieved in the very first HoQ, but then it wouldn’t have been that clearly arranged. It has to be decided individually which procedure is suited best regarding time, profit and usage.
The HoQ concerning the tread pattern offers the possibility to plot the target values, the direction of improvement and the matrix of the rooftop.
Quality Characteristics of the component “tread pattern”
� cap-material: % silica
� cap-material: % flexibiliser
� cap-profile: share of negative tread (profile)
� cap-profil: profile depth
� base: density
� base: hardness
� shoulder: angle
� shoulder: hardness
� ...
Quality Characteristics:
Identification of Quality Characteristics (QC)
In the following work step the Quality Characteristics of the tread pattern will be identified. They represent measurable criteria for the fulfilment of the Customer Requirements and can be used for the technical implementation.
The Rooftop Matrix is used for the identification of interactions between the various Quality Characteristics.
These can be of different characteristic:
- Strong Positiv interaction ++
- Positiv interaction +
- Neutral no inscription
- Negative interaction -
- Strong negative interaction --
Example:
Silica and flexibiliser are both parts of the cap material. The procentual quota of the flexibiliser should stay the same but the quota of silica should be raised. Because the material is a mixture of both a procentualchange of one part influences the quota of the other part which means there is a strong negative interaction (shown by - -).
Symbols are used for documenting the correlation between Customer Requirements and Quality Characteristics in the matrix. Three intensity levels are distinguished when using the tabular description.
A circle with a dot in the middle represents for a very strong correlation. For the calculation of the characteristic importance a numeric value of 9 is used.
A medium strong correlation is represented by simple circles (value: 3).
Triangles are used to point out a weak correlation (value: 1).
Usually the correlations are determined during a discussion of experts from all concerned business divisions like design, planning, construction, manufacturing, assembly and quality assurance. According to that only a relatively rough gradation can be used when evaluating the correlations. The term “intensity of correlation” is comparable to the same term, which is used in the fields of statistics.
Than the absolute and the relative importance can be calculated. The absolute importance is calculated by summing up the product of the weight and the value of correlation within a column. The relative importance can be calculated analogue to the calculation of the weight used for the pair wise comparison. (See next page)
Exercise: Calculate the absolute and relative importance of the quality characteristics.
A strong correlation exists, if the change of a Quality Characteristic leads to a systematic change of the customer satisfaction (in the sense of the fulfilment of a certain Customer Requirement).
The algorithm for calculating the absolute importance of Quality Characteristics can be described as the following:
Importance of the first Customer Requirement (CR#1) multiplied with the correlation to the first Quality Characteristic (QC#1) plus the importance of CR#2 multiplied with the correlation to QC#1 plus CR#3 x QC#1 plus .... This proceeding has to be repeated for every column (quality characteristic).
The relative importance is calculated analogue to the pair wise comparison :
i = column j = row
Gj = weight of Customer Requirement
Bij = strength of correlation with Quality Chrateristic
for a range of 1 to 5
for a range of 1 to 10
Wi : weighting; Si : sum of correlating line; Smax : maximum line sum;
An additional goal of using QFD in product planning is to determine target values for a detailed development. For this purpose the Quality Characteristics and the Customer Requirements have to be analysed accurately by regarding realisation aspects and their respective market potential.
The way which is usually taken to find out a solution is to compare products directly (benchmarking) and to survey customers about how the regarded products fulfil their requirements.
So the HoQ has to be completed by a customer based and a technical based comparison. Such comparisons in QFD mostly use an integer scale from 1 (“particularly well“ resp. “fulfilled“) to 6 (“particularly bad“ resp. “not fulfilled“). The results of a customer survey and a technical comparison are displayed above and on the following page.
The HoQ of process planning determines the correlation between process steps and Quality Charactericsof the components.
Analogue to the very first HoQ it is not possible to set up target values or the direction of improvement. Hence filling in the rooftop matrix is not feasible.
The production planning provides the correlation between the setting parameters and the Quality Characteristics of the components. This HoQ can be filled in completely. Another thinkable possibility might be to combine the third and fourth house in only one House of Quality.