In evaluation tables several variants are evaluated based on different criteria. Examples are tables with criteria for the selection of a location of an industrial company, cost-benefit analyses, or the evaluation of concept variants according to VDI 2222 and VDI 2225 sheet 3. There the concept variants arefirst evaluated regarding their technical properties, then they are evaluated regarding their economical properties and then two evaluation tables are created.
Atfirst, we want to show you the procedure as it is defined in the standards. Then we want to recommend a few deviations from the standard. Please speak with your supervisor or customer in advance, which procedure and table design shall be used.
In VDI 2222, the concept finding for technical products is standardized. It has the phases
– planning,
– concept-finding (list of requirements), – function analysis (specification),
– concept (concept variants with technical and economical evaluation), – draft (assembly drawing),
– optimization,
– refinement (single part drawings), and – production of a prototype.
Beside many other examples the evaluation of a water purification plant is introduced.
The concept variants should get meaningful names, which your readers can keep in mind easily. The header should be emphasized with bold type as here in the book or with Table 3.4 Morphological box: subdivided sub function
Sub function Solutions of the sub functions
1 2 3 4 5
A …
B Motor and cylinder cooling
Air cooling Water cooling
Ring cooler
Tube cooler
Forced-circulation cooler
Flow cooler
C …
gray shading. But now let us look at the individual steps of the evaluation, Tables 3.5,3.6, and Fig.3.8.
The strength s of the variants, which results from the x, y-coordinates of the four variants, is now drawn as points s1, s2, s3, and s4 into the so-called s-diagram. The ideal solution siis drawn at the position x = 1.0 and y = 1.0. Then a straight diagonal line from the lower left to the upper right corner is drawn that runs across the whole diagram. The best concept variant, here variant 3, is the one which can be found in the far right and far top of the diagram.
All evaluation tables must give exact information atfirst sight. If you read the table and the legend, it must be clear, which criteria have influenced the evaluation how strong and which variant could gain how many points. Basic rule: The evaluation table shall not be a mental exercise. Therefore, it must always be explicitly stated which variant has“won”: the variant with the most points or—much more seldom—the variant with the least points.
For an appropriate evaluation you often have to give different evaluation criteria a different level of influence on the final rating of the concept variants. To express this different level of influence, weighting factors have been introduced. You have to multiply Table 3.5 Technical evaluation properties of a water purification plant (acc. to VDI 2222)
Technical evaluation properties of the water purification plant
Points for variants 1 to 4
Var. 1 Var. 2 Var. 3 Var. 4 ideal
Blockage risk 2 3 4 3 4
Emission of smell 3 3 3 3 4
Emission of noise 3 3 2 3 4
Required space 1 2 3 2 4
Operational safety 3 3 4 2 4
Sum 12 14 16 13 20
Technical rating x 0.60 0.70 0.80 0.65 1
Table 3.6 Economical evaluation properties of a water purification plant (acc. to VDI 2222) Economical evaluation properties
of the water purification plant
Points for variants 1 to 4
Var. 1 Var. 2 Var. 3 Var. 4 ideal
Excavation 2 3 4 3 4
Concrete work 3 3 3 3 4
Expenses for pipes andfittings 3 3 2 3 4
Assembly costs 1 2 3 2 4
Maintenance costs 3 3 4 2 4
Sum 12 14 16 13 20
Economical rating y 0.60 0.70 0.80 0.65 1
the simple point value with the appropriate weighting factor to get the total point value of the variant regarding the current evaluation criterion. Adding the total point values of all criteria leads to the sum total value of the current variant, which is listed in the evaluation table in the lowest row.
Table3.7is a concrete example, how an accurate evaluation table looks like. It has been created to evaluate several variants of the chassis of a boat trailer.
Please look at the legend. Atfirst, all abbreviations are explained. Then the meaning of the evaluation factors is defined. So, the reader can comprehend, how each sum total has been computed. To avoid logical mistakes, you should apply the following principles when you give evaluation points:
– The given points are“positive points”.
– High simple point values mean high value and high benefit for the users.
– Due to the multiplication with the simple point value high weighting factors result in a high level of influence of the current evaluation criterion on the rating result (=the sum total, the added total points) of the concept variant.
In the example “chassis of a boat trailer”the evaluation criteria self-weight and load carrying capacity must be weighted differently: If the load carrying capacity raises, the (positive) point value increases. But if the self-weight raises, then the (positive) point value must decrease! Written like a formula the situation looks as follows:
si
s3 s4
s2 s1 1
0,9 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1
0 0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1
Fig. 3.8 Technical-economical evaluation of the concept variants of a water purification plant (from VDI 2222)
Point values of evaluation criteria with high and low benefit
load carrying capacity" ) benefit for the users" )simple point value"
self-weight " )benefit for the users# ) simple point value#
We can see that there are obviously two different cases.
Two weighting types in evaluation tables
1st case: parallel point values measuring value of criterion"and points"
(here: load carrying capacity)
2nd case: opposite point values measuring value of criterion"and points# (here: self-weight)
Table 3.7 Example of a technical evaluation table for the chassis of a boat trailer Evaluation criterion Weight 1 axis, 2
wheels
2 axes,4 wheels
2 axes, 3 wheels, 1 wheel steerable
2 axes, 4 wheels, 1 axis steerable
w SP TP SP TP SP TP SP TP
Self-weight 10 3 30 2 20 2 20 1 10
Assembly 6 3 18 2 12 1 6 1 6
Price 10 3 30 2 20 1 10 1 10
Ease-of-use 8 3 24 2 16 2 16 2 16
Design 4 3 12 2 8 1 4 1 4
Chassis 8 4 32 3 24 3 24 3 24
Load carrying capacity 8 2 16 4 32 3 24 4 32
Sum total 162 132 104 102
Legend
w = Weighting factor SP = Simple points TP = wSP = Total points w = 2, very low impact w = 4, low impact w = 6, medium impact w = 8, high impact w = 10, very high impact SP = 0, not suited SP = 1, with deficiencies SP = 2, satisfactory SP = 3, good SP = 4, very good
Whether the points must be distributed parallel with the measuring value of the cri- terion or opposite to it causes many logical mistakes in evaluation tables. It is a problem of language logics, which is anyway harder to understand than mathematical logics for many people.
Often several of these evaluation tables follow one another. Then every table must get its own legend, so that every table is readable on its own and unnecessary turning of the pages is avoided.
A few keywords are a signal for opposite point values. Examples: High effort or expenses are the opposite of high benefit and have to be weighed with opposite point values. Raising cost expenditures result in decreasing simple points. A raising initial training effort, to learn how to handle a technical product is treated in this way, too. And finally a raising learning effort also results in a falling simple point value.
When distributing the simple points think of the users or customers and not of the manufacturer or service provider! Then it is easier to decide, whether parallel or opposite point values are required.