Creating Something from Nothing:

Working with Synthetic Files

ACCOLEDS /DLI Training: December 2003

Chuck HumphreyUniversity of Alberta

Outline

Types of microdata files Which microdata file to use Providing services for synthetic files

This presentation is a modification of a workshop that Bo Wandschneider and I presented at the May 2003 National DLI Training program.

Types of Microdata Files

• Confidential Microdata Products• Master Files• Share Files

• Public Access Microdata Products • Public use anonymized microdata

(PUMFS)• Synthetic Files

Microdata Products

Microdata• raw data organized in a file where the

records or lines in the file are observations of a specific unit of analysis and the information on the lines are the values of variables

• requires some form of processing or analysis to be used

Microdata Products

CCHS 0000015959922220611230721241433296101121222222112222222223060.75021.62211022960104000096009604000000000022666626666666666666666666011666666666311142222221222266226122222669662266621222222212136666666666666666666669996666152122222222222666666666666666666666666666666666666666666666666666666666666666666966666666666666666966666666666666666666666666666666666666000.4001.0000.0000.0000.1000.1001.71122222222222222222222226997999669966996699669966996699669966996699669966996699669966996699669966996699669966996699660101300.0100032396969696966666662966666662696969666666666662696011111101101.0009696969666961922221033969969969666969966960599666666666266661111222222210501100000100000000000000166666666666666610020002000.006666969669966996669669666666607101122666296666666969609669696000.009696662621112441100412102119630401161245060522333200224.170000023535951221521330523226642101103266666666266666666619045.90999.66221002960405010203009600000000000012212226666666666666666666061266666666114132111221122266226222222669662266622222222221116666666666666666666669996666116666666666666152212222222222222666666666666666666666621512222222222221222212226026666666666666666966666666666666116666666666666666666666000.1001.0001.0001.0001.0001.0005.12222221222222222222221226996699669966996699660602996699669966996699669966996699669966996699669966996699660032996699660101100.8102112301960705066666662966666662696969666666666662696021141201100.4599696969666969613222963969969969666969966960699666666666266662226666222229696699699699699699699699612666666666666639969962000.006666969669966996669669666666696969696666296666666969609669696000.009696662612631340000312696669669966663234040122333200317.04

Confidential Microdata

Master Files• These files contain the fullness of detail

captured about the unit of observation. The information in these files could identify the individual who provided the original information and, therefore, are considered confidential.

Confidential Microdata

Master File – Example

Confidential Microdata

Master File – geography

Confidential Microdata

Master File - fullness of data

Confidential Microdata

Master File - fullness of data

Confidential Microdata

Master File - fullness of data

Confidential Microdata

Share Files• these are confidential files in

which the respondents have signed a consent form permitting Statistics Canada to allow access to their information for approved research.

• Used with NPHS and NLSCY

Public Access Microdata

Anonymized Microdata• these microdata are specially prepared

to minimize the possibility of disclosing or identifying any of the cases or observations

• the original data from the master file are edited to create a public use microdata file

Public Access Microdata

Steps in Anonymizing Microdata• removal of all personal identifiers• include only gross levels of

geography• collapse detailed information into

fewer general categories or cap values

• suppress the values of a variable

Public Access Microdata

Statistics Canada PUMFs• only available for select social

surveys that undergo a review of the Data Release Committee, an internal Statistics Canada committee;

• no ‘enterprise’ public use microdata;

Public Access Microdata

Statistics Canada PUMFs• almost all are cross-sectional, that

is, represent data collected at one point in time;

• longitudinal data are difficult to anonymize while maintaining any useful information.

Public Access Microdata

PUMFs – personal identifiers

Public Access Microdata

PUMFs – collapsed data

Public Access Microdata

PUMFs – suppressed data

Public Access Microdata

Synthetic Files• These microdata do not contain

actual ‘real’ cases but are pseudo-cases that for some surveys, provide aggregate results close to the ‘real’ cases

Public Access Microdata

Synthetic Files• They have been prepared to

create analysis runs with the master file without possibly disclosing or identifying any of the cases

Public Access Microdata

Synthetic Files• The results are not to be reported,

but are strictly to be used to prepare analyses of master files;

• Usually associated with longitudinal files.

Public Access Microdata

Steps in creating Synthetic Files• Observations are transformed• No records actually exist• Keep fullness of variable

description• How the files are made is kept

confidential

Public Access Microdata

Synthetic Files – CCHS Cycle 1.1

PUMF Synthetic

ObsLrecl

130880 841

65101 1778

Var 614 1164

Implications for Analysis

What are the implications in doing analysis with these different types of microdata files?

Implications for Analysis

Master File• All observations• Has the most variables with the

most detail• Lots of geography and personal

characteristics• Little grouping or capping of

categories

Implications for Analysis

Master File• Restricted access: only available

to authorized Statistics Canada employees, which includes ‘deemed employees’;

• Use of the analysis is controlled through a contract;

Implications for Analysis

Master File• Includes linkage variables across

files within a study, e.g., NLSCY linkage among the files for different units of analysis (kids, parents, teachers).

Implications for Analysis

Public Use Microdata (PUMF)• Valuable content for a tremendous

amount of research;• Where issues arise is when smaller

area geography is desired; rare subpopulations are being studied; or the variables that are needed have been used to anonymize respondents;

Implications for Analysis

Public Use Microdata (PUMF)• Licensed product: agree to certain

terms of use;• No linkage to multiple units of

analysis, except for a few exceptions (e.g., GSS Time Use and Family);

Implications for Analysis

Synthetic Files“Looks like a duck and quacks like a duck”, but it isn’t a duck or any other type of fowl.

Implications for Analysis

Synthetic Files• Looks like master files• Lots of observations• Lots of variables• Little grouping or capping of categories• Lots of geographic detail

Synthetic Files

Precautions• Results not authentic – but may be

close in the aggregate for some synthetic files;

• Use for testing analysis setups only;• Still need the master files for

publishable results.

Where do we get Access?

Master File• Restricted access governed under the

Statistics Act;• Remote Job Submission (a.k.a, RDA)• Research Data Centres

• Apply to SSHRC to obtain a peer-reviewed proposal and STC for security clearance.

Where do we get Access?

Public Use Microdata Files (PUMF)• Get from DLI• Analyze where it is convenient • Can use a variety of analysis

software, including SAS, SPSS, Stata, HLM, LISREL, etc.

Where do we get Access?

Synthetic Files• Author Divisions ‘may’ create it• Most relevant when dealing with

new Panel Data, but not necessarily, e.g., the Census has potential

• NPHS & CCHS synthetic files on DLI FTP site

Where do we get Access?

Synthetic files• Work locally with the file• Build SAS and SPSS setups

Which File is Appropriate?

• 1st stop is still the PUMF;• This file has the easiest access for us;• Probably meets the needs of most

patrons;• Not as administratively burdensome as

synthetic or master file;• Perfect for clients just looking for ‘data’ –

courses in quantitative analysis;

Which File is Appropriate?

• If more detail is needed, refer to the Master File Documentation;

• Inform patrons that the cost of use is higher, both in terms of accessibility and analytical requirements;

• Interest most likely to come from grad students and ‘experienced’ researchers

Which File is Appropriate?

• Download the Synthetic files from DLI

• Make them aware of problems with synthetic files – RESULTS ARE NOT PUBLISHABLE

• Encourage them to submit an application for RDC access – there is a time lag

Which File is Appropriate?

• Some of you may work with patron using synthetic files before passing her/him off to RDC.

DLI Contacts can provide four basic services with synthetic files.

• Build SPSS and SAS system files from the raw synthetic data files that are distributed through DLI;

• Provide information about the use of Remote Job Submission and RDC’s;

Services for Synthetic Files

• Assist with finding variables in the synthetic files;

• Provide instruction about ways of capturing SPSS or SAS code from “dummy” analysis runs with the synthetic files. It is this code that is submitted to STC through remote job submission.

Services for Synthetic Files

1. Building SPSS and SAS system files for synthetic data• The CCHS synthetic data are distributed

as a raw ASCII file with accompanying command files for SPSS and SAS

• Separate synthetic data files exist for the master file setup and for bootstrapping analysis

Services for Synthetic Files

1. Building SPSS and SAS system files for synthetic data• The synthetic data for the CCHS Cycle

1.1 has 1,164 variables and 65,101 fabricated cases. Creating the SPSS and SAS system files from this file is not difficult, but it does take time. DLI Contacts may wish to create these products for their patrons.

Services for Synthetic Files

2. Information about Remote Job Submission (RJS)• The author divisions supporting RJS have

established their own guidelines and have different operating procedures. Not all divisions supporting longitudinal surveys currently support RJS (e.g., SLID).

• Therefore, there is a need to track down this information for our patrons.

Services for Synthetic Files

2. Information about Remote Job Submission (RJS)• For example, the sources for information

about RJS include the Centre for Education Statistics:

http://www.statcan.ca/english/edu/rda/index.htm

Services for Synthetic Files

2. Information about Remote Job Submission (RJS)Where do you find this information?• Ask the DLI Team via the DLI List• The EAC has asked for a description of

RJS on the DLI website, which should be on the DLI Team’s to-do list

Services for Synthetic Files

2. Information about Research Data Centres

• The collection of master files available through RDC’s is listed on the STC website for RDC’s

• Each RDC has its own website describing its services

http://www.statcan.ca/english/rdc/index.htm

Services for Synthetic Files

3. Data Reference for the content of the synthetic files

• Helping researchers identify variables over longitudinal files is an important service

• Need to keep the unit of analysis straight• Need to understand the mnemonic naming

convention for variables over cycles• Develop indexing aids for you and your

patrons

Services for Synthetic Files

4. Provide helpful tips for preserving the code from “dummy” analysis runs in SPSS and SAS

• Researchers will run analyses on the synthetic file to generate the code that they will subsequently email for Remote Job Submission

• Providing information about how to do this easily will be helpful to your patrons

Services for Synthetic Files