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A12. Calculating averages
When calculating average values for the performance characteristics of several plants each of which contains multiple steps, there are two different procedures that may be employed.
Figure 8.
Vertical averaging method employed in this report to calculate average values. See
text for explanation.
Figure 9.
Horizontal averaging procedure. See text for explanation.
The first method, sometimes referred to as vertical averaging, is illustrated in Figure A10. In this method each complete production sequence is evaluated separately and the final result is calculated as the average of the results from the individual production sequences weighted by the output from each production sequence. Averages can be produced for intermediate production steps as shown but these intermediate averages do not affect the final result.
The second method, sometimes referred to as horizontal averaging, is illustrated in Figure 9. In this method, an average is produced for each intermediate step and this average is then used in all of the production sequences to calculate the next step down-stream. The procedure is repeated for the full length of the sequence as shown.
When the performance characteristics of the intermediate steps are similar in all of the production sequences, the results produced by the two methods are expected to be very close to each other. However, when there is a significant divergence in performance characteristics, the average values can be significantly different and these differences can be amplified if the production levels in the different sequences are markedly different.
In the present report, the vertical method of averaging has been used wherever applicable since this most closely approximates to actual practice. It possesses the additional advantage of allowing individual companies to see the precise performance of their own operations; with the horizontal method of averaging, this is not possible because each average involves practices which reflect the behaviour of other companies.
It is perhaps worth noting that the need to choose between these two methods of averaging has only occurred recently. When information about processes was scarce, data were often only available from a single site and this information was usually used in all applications. Thus for many years information on the production of ethylene was based on information from one or two plants. In practice however, there are almost 100 plants in Europe alone and all of these plants do not feed all processes using ethylene. It therefore becomes important to identify, as far as possible, the site specific data for each production sequence, if the results are to be representative of actual practice.
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