The new approach as implemented in a test version ofτ-argus, starts with con- structing the same cover table as the one used in the naive approach. However, whilst the naive approach would protect the complete cover table, the new ap- proach protects only the parts of the cover table that belong to any of the tables of the core set. The parts of the cover table that appear in none of the tables in the core set, will not be protected. Essentially this means that in the breakdown of the cover table into non-hierarchical subtables, some of these subtables will not be protected. See [2] and [3] for more details. Table 4 shows the results for this approach. The time needed to find this solutions was about two minutes.
Table 4.Results of the new approach
Number of cells Costs (×106) Table name Total Empty Primary Secondary Total Secondary
T1.1 545 4 54 25 6.878 52
T2.1 1,078 66 122 156 8,820 273
T2.2 549 5 21 40 11,789 36
T2.3 and T3.1 1,326 17 100 118 16,194 229
6 Conclusions
As has been shown in the previous sections, there are several approaches to choose from, when applying cell suppression as a disclosure control technique to the core set of SBS tables as defined in this paper.
72 P.-P. de Wolf and A. Hundepool
The naive approach is obviously the easiest way to proceed. However, it often leads to over protection. Cells that do not appear in any of the tables of the core set, will be protected as well. This will often lead to additional suppressed cells that do appear in one or more of the core tables. Indeed, table 2 shows that the total information loss in terms of the number of suppressed cells as well as in terms of the sum of the suppressed cell values is larger than for the other two approaches.
The other two approaches are taking care of that problem by protecting only the published tables. The traditional and the new approach both lead to the same suppression patterns. For the instance used in this paper, it turns out that only a limited number of secondary suppressions need to be carried over in the traditional approach. Indeed: only in the process of going from table T1.1 to T2.1 six secondary suppressions needed to be carried over. Moreover, table T3.1 turned out to have only one single primary unsafe cell. However, in general this would not be the case.
For the same reason, no iterative procedure was needed in the traditional approach: assigning the status ‘protected’ to the safe cells that were carried over, still made it possible for τ-argus to find feasible patterns. Hence, in our instance, the intensity of the manual interaction was not much for the traditional approach.
Another aspect that in theory could influence the intensity of the manual interaction, is the order in which the tables are protected. In our paper we made use of the suggestions made in [1].
The main advantages of the new approach can be summarized as:
– It is not necessary to think about the order in which the linked tables should be protected.
– No additional manual interaction is needed to carry over suppression pat- terns between tables.
– Manual backtracking will never be necessary.
– The amount of overprotection is limited.
A disadvantage of the new approach is that it can only be used when the set of linked tables can be viewed as part of a so called cover table of up to 4 dimensions. This restricts the number of situations in which the new approach is applicable. However, in practice a cover table of 4 dimensions often suffices. In the situations where the new approach is not applicable, the traditional approach would be a good alternative.
Concluding we would argue that the new approach as discussed in section 5 is to be preferred. Since we only applied the three approaches to a single instance, additional research should be made on other instances to back our conclusions.
References
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