Perturbation

Small values in cross tabulation results have the potential to reveal confidential information. Perturbation can be used to reduce that risk by adjusting cell values by small amounts.

A key (referred to as the cell key) is generated for each cell in the table based on the RKEYs for each unit record contributing to that cell value. The cell key and the cell count are then used as parameters to access the perturbation table, which is pre-defined and deployed for each database. The value obtained from looking up this perturbation table is then added to the cell value, to provide a perturbed result.

If you need to use perturbation in your deployment, please contact Space-Time Research support (support@spacetimeresearch.com) for advice on the appropriate approach for your processing needs.

Prerequisites

To use perturbation, you must have:

Rkeys in the unit records.
A perturbation table (PTable) file for each database, defined in a CSV file saved in the same location as the database file, but with the extension .pert.

With weighted databases, perturbation is not designed to work by itself. You may need another Data Control API module to calculate perturbed weighted values.

Module Properties

The Perturbation module has the following properties you can configure.

Property	Description
RKEY	Set this to `true` to use the Rkeys in the unit records.
FREQ	Set this to `true` to perturb cell values based on the contribution count rather than the cross-tabulation (cell) value.
SmallN	An integer to be used in deriving the lookup values for the perturbation table. Its value can be 10 or below. The default value is 10.
PTableSize	The size of the perturbation table. The default value is 30.
BigN	An integer to be used as the modulo base when adding RKeys. Must not exceed 2^32 (4294967296). The default value is 4294967296.
ConfidentialityModule	If this is set to false, RKEYs can be collected for all statistical function summations. If this is set to true, SuperSERVER will block the query due to the fact that the statistical functions cannot be protected by Data Control.
Message	A message to be displayed to users in the client.
PTable	This setting is available from version 8.0.4.16 onwards. It is not available in SuperSTAR 8.0 GA. The location of the perturbation file for this database. If you do not set a value for the PTable property, then by default SuperSERVER expects this file to be saved in the same location as the database file, but with the extension .pert instead of .sxv4. For example, if the database file is C:\ProgramData\STR\SuperSERVER SA\databases\RetailBanking.sxv4 then the perturbation file is expected to be located at C:\ProgramData\STR\SuperSERVER SA\databases\RetailBanking.pert If you want to use a different location for the file, then you can set the value of PTable to the location of the .pert file. You can either use an absolute path or a relative path (relative to the SuperSERVER program data directory, which is C:\ProgramData\STR\SuperSERVER SA if you installed to the default location). Any backslashes in the path will need to be escaped with an additional backslash (forward slashes can also be used but do not need to be escaped). For example: CODE `method "Rule" perturbation addproperty PTable "C:\\ProgramData\\STR\\SuperSERVER SA\\databases\\my-ptable.pert"`

Apply the Plugin to a Database

Login to SuperADMIN and create a new method:
CODE
```
> method addmethod perturbation_method
```
Add the Perturbation Data Control plugin to the method:
CODE
```
> method perturbation_method adddcplugin perturbation Perturbation
```
The perturbation module is named Perturbation. It is internal to SuperSERVER, and the naming is case sensitive.

Set the plugin properties:

CODE

> method perturbation_method perturbation addproperty RKEY "true"
> method perturbation_method perturbation addproperty FREQ "true"
> method perturbation_method perturbation addproperty "SmallN" "10"
> method perturbation_method perturbation addproperty "PTableSize" "30"
> method perturbation_method perturbation addproperty "BigN" "4294967296"
> method perturbation_method perturbation addproperty ConfidentialityModule "true"
> method perturbation_method perturbation addproperty Message "Data has been perturbed"

Assign the method to a database (in this example we are assigning the method to a database with the ID bank):

CODE

> cat bank addmethod perturbation_method

You can review the method details using the command cat <database_id> methods details <method_id>:

CODE

> cat bank methods details perturbation_method
[ Method : perturbation_method (id:perturbation_method) (type:mandatory) ]
    [ Common ]
    [ DCPlugin : Perturbation (id:perturbation) (priority:1) ]
        [ RKEY : true ]
        [ FREQ : true ]
        [ SmallN : 10 ]
        [ PTableSize : 30 ]
        [ BigN : 4294967296 ]
        [ ConfidentialityModule : true ]
        [ Message : Data has been perturbed ]

Continuous Perturbation

The original module for perturbation is for the perturbation of discrete variables only. Continuous variable perturbation applies to continuous variables such as income. It can operate on both weighted and unweighted databases.

To use continuous perturbation, you must have:

Rkeys in the unit records.
An Ftable stored in a CSV file:
- This file must have exactly 2 columns.
- The first column represents the rank (with respect to TOPN) and must be an integer value, starting at 1 in the first row and incrementing for each additional row (1,2,3, etc).
- The second column represents the scaling factor and must be a number (can be a floating point value).
A CTable stored in a CSV file:
- This file must have exactly 256 rows and at least 32 columns.
- All values must be numbers.

Apply the Plugin to a Database

The following examples show how to configure continuous perturbation for unweighted and weighted databases:

Unweighted Database

Configure the method:

CODE

> method addmethod cont_perturbation_method
> method cont_perturbation_method adddcplugin perturbation Perturbation
> method cont_perturbation_method perturbation addproperty RKEY "true"
> method cont_perturbation_method perturbation addproperty FREQ "true"
> method cont_perturbation_method perturbation addproperty TOPN "2"
> method cont_perturbation_method perturbation addproperty TOPN_RKEY "2"
> method cont_perturbation_method perturbation addproperty SMALLC "5"
> method cont_perturbation_method perturbation addproperty "FTABLE" "C:\perturbation\ftable.csv"
> method cont_perturbation_method perturbation addproperty "CTABLE" "C:\perturbation\ctable.csv"

Notes:

TOPN_RKEY is independent from TOPN but must be set to the same value. SuperSERVER processes a list of measure and rkey pairs, ranks the rkeys based on the descending order of the associated measure, and then picks up the top n rkey.
The SMALLC property is optional. It defaults to 5 if not specified.
If you do not set the FTABLE or CTABLE properties, then they default to a file located in the same location as the SXV4 database, with the extension .ftable or .ctable.

Assign the method to a database (in this example we are assigning the method to a database with the ID bank):
CODE
```
> cat bank addmethod cont_perturbation_method
```

Weighted Database

Configure the method:

CODE

> method addmethod cont_perturbation_method
> method cont_perturbation_method adddcplugin perturbation Perturbation
> method cont_perturbation_method perturbation addproperty RKEY "true"
> method cont_perturbation_method perturbation addproperty FREQ "true"
> method cont_perturbation_method perturbation addproperty TOPN "2"
> method cont_perturbation_method perturbation addproperty TOPN_RKEY "2"
> method cont_perturbation_method perturbation addproperty TOPN_MAIN_WEIGHT "2"
> method cont_perturbation_method perturbation addproperty SMALLC "5"
> method cont_perturbation_method perturbation addproperty "FTABLE" "C:\perturbation\ftable.csv"
> method cont_perturbation_method perturbation addproperty "CTABLE" "C:\perturbation\ctable.csv"

Notes:

TOPN_RKEY and TOPN_MAIN_WEIGHT must be set to the same value as TOPN.
The SMALLC property is optional. It defaults to 5 if not specified.
If you do not set the FTABLE or CTABLE properties, then they default to a file located in the same location as the SXV4 database, with the extension .ftable or .ctable.

Assign the method to a database (in this example we are assigning the method to a database with the ID bank):
CODE
```
> cat bank addmethod cont_perturbation_method
```