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of the weighting procedure, but confidence in the results can be increased by carrying out a sensitivity analysis. This is done by looking at the effects on ranking caused by changes in the weighting. Select the criteria most open to subjective interpretation. Change the weights of those criteria slightly, recalculate the final scores and re-rank the communities. Comparisons between the ranking order for different scoring and weighting scenarios will indicate the level of confidence that can be given to the results. Minimal changes in ranking order indicate a high degree of confidence in the results. Conversely, wide variations in ranking will indicate that the results should be treated with care. Table 3.10 shows the effect of reducing the weighting for 'tourist impact' from 3 to 2 and increasing the weight for 'unit cost' of construction from 2 to 3 (first re-ranking) and setting all the weights to unity (second re-ranking). Table 3.10 shows that, with the exception of town 'J' the change in weighting numbers has very little effect on the overall ranking of the communities; therefore, the results can be accepted with confidence. The actual numbers are of little relevance, only their relative size. |
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A matrix is an excellent method for deciding the order in which communities should be provided with sewerage. By selecting communities using criteria based on need, benefit and cost, the steps in the process can be justified and the final results can be described mathematically. The method is not absolute. It will not select the single most needy community for sewerage. It will, however, segregate a group of communities into those which can cost-effectively obtain the greatest benefit from sewerage and those with less need or requiring greater subsidy. There will normally be a group in the middle (as has occurred in the example) for which the costs and benefits from sewerage make its provision optional. The individual ranking within that group is irrelevant. |
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The selection process described here is primarily technical and financial. Environmental and health considerations also have a place in the selection process. These could be included in the matrix or considered separately. Reference may be made to World Bank (1991) for further details of these areas. |
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