Confidentiality Rule

The Confidentiality Rule module provides two techniques for applying confidentiality to cell values:

• Rounding - Values that meet certain criteria (typically very small values) are randomly rounded up or down to a different value.
• Cell Suppression - Cells that meet certain criteria, such as those with a small number of contributors, are suppressed entirely.

Rounding Rules

Rounding rules adjust values up or down to a defined base.

Rounding is only intended for integer counts. If random rounding is applied to cells with real values, the results will always be integers.

The confidentiality rule module includes the following rounding rules:

RuleDescriptionEffect of Applying Rule
0-3 rounding

Randomly changes any values of 1 and 2 in the table to either 0 or 3.

• 0 is not changed
• 1 is changed to 0 with probability 2/3
• 1 is changed to 3 with probability 1/3
• 2 is changed to 0 with probability 1/3
• 2 is changed to 3 with probability 2/3
• Values of 3 and above are not changed
Random roundingAll values are rounded up or down to base 3 after tabulation.
1-4 rounding

Randomly changes any values in the table between 1 and 4 inclusive to a random number between 1 and 4 with equal probability distribution.

• 0 is not changed
• 1 is changed to 1, 2, 3 or 4 with equal probability
• 2 is changed to 1, 2, 3 or 4 with equal probability
• 3 is changed to 1, 2, 3 or 4 with equal probability
• 4 is changed to 1, 2, 3 or 4 with equal probability
• Values of 5 and above are not changed.

This rule is similar to random rounding in that all values are rounded. However, the amount by which values are rounded depends on the size of the value.

• 0 is not changed
• Values from 1 to 18 are randomly rounded to one of the two nearest multiples of 3
• Values of 19 randomly round to 18 or 20
• Values from 20 to 100 randomly round to one of the two nearest multiples of 5
• Values greater than 100 randomly round to one of the two nearest multiples of 10.

Rounding output is determined only by the input data, so the same randomly rounded results will be produced every time if the same table data and dimensions are used, and each recode has the same number of categories. Variations in results may occur when fields are added to tables in a different order.

Rounding and Totals

Any rounding of total values as a result of these rules is only applied after the total has been calculated from the unrounded microdata. This ensures that totals do not suffer from accumulated rounding errors.

For example, when the 0-3 rounding rule is in use, a total value will at most only be out by 2 integer values from the unrounded total.

Some of these rules can also be run directly in the SuperCROSS client. When the rules are applied client side in SuperCROSS, the totals are calculated based on the rounded values, and therefore may suffer from accumulated rounding errors. If it is important to you to eliminate accumulated rounding errors in totals then you are recommended to use the Data Control module for rounding rather than client side rules.

Rounding and Client Side Computations

Rounding from Data Control is always carried out first, before any client side computations that you might have configured (such as sum derivations, classification derivations, and percentages). The confidentiality rule module provides the client with the already rounded data, and the client uses this to perform its calculations.

Examples

The following examples show the effect of the rounding rules on the Retail Banking sample database.

Click the images to view a larger, full size, version.

SuperCROSS

 No rounding: 0 – 3 rounding: Random Rounding: 1-4 Rounding: Graduated Rounding: SuperWEB2

 No rounding: 0-3 Rounding: Random Rounding: 1-4 Rounding: Graduated Rounding: Cell Suppression Rules

Cell suppression rules allow you to hide the results of any cells that meet certain criteria:

RuleCell Values Will Be Concealed If...For Example...
Top Contributors

The biggest n contributing values make up more than a certain percentage of the cell value.

If you set:

• The number of contributors to 3; and
• The percentage to 75%

then cell values will be concealed if the biggest 3 contributors make up 75% or more of the total cell value.

Any cell with (in this example) 3 or fewer contributors will obviously always be concealed.

This is because if there are only 3 contributors, then clearly all 3 of them are the "biggest 3" contributing values, which must therefore make up 100% of the cell's value.

FrequencyThe number of contributors to the cell value is less than or equal to the specified frequency.If you set the frequency to 30 then any cells with under 31 contributors will be concealed.
ThresholdThe value is equal to or below a certain threshold.If you set the threshold to 300, any cell values of 300 or below will be concealed.

The suppression rules do not apply to cells whose value is 0. As the default replacement string for suppressed cells is ..C this means that users will be able to tell the difference between cells whose value is really 0 and cells that have been suppressed.

To avoid this, you may wish to change the replacement string for suppressed cells. For example you can choose to replace the cell value with 0 instead of using a confidentiality string. See Configure the Confidentiality Rule for more details on this configuration adjustment.

Threshold Rule Example

 SuperCROSS, no disclosure control: SuperWEB2, no disclosure control: SuperCROSS table concealing cells with values of 500 or below: SuperWEB2 table concealing cells with values of 500 or below: 