Definition: p% rule

Data protection

This  rule states that a cell, contained in a table of magnitudes, is at risk if the value for any contributor can be calculated to within a given percentage. Cells that are identified as sensitive must be suppressed or otherwise avoided (through table design).
The p% rule calculates the distance (as a percentage) between the estimated value and the true value for the largest contributor in a table, as follows (see source for mathematical formula):

X is the value for the largest contributor, ^X = Total - Y is the estimate of X, and Y is the value for the second largest contributor. If the value of p is less than the p% threshold then the cell is deemed sensitive. If a cell is sensitive then aggregation or suppression (with secondary suppression) must be applied to the table. If a contributor’s value is negative, take the absolute value before calculating p.

The value of the threshold is confidential and is not to be made public.
Source:
Statistics New Zealand, “Microdata access output guide”, Wellington, New Zealand, October 2013
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