Creating Something from Nothing: Working with Synthetic Files

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Creating Something from NothingWorking with
Synthetic Files

• Bo Wandschneider University of Guelph

DLI Training April 2004, Kingston
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Outline

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

This presentation is a modification of a workshop
that Chuck Humphrey and I presented at the May
2003 National DLI Training and a Presentation
Chuck presented at Accoleds DLI training, 2003.
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NLSCY

• The National Longitudinal Survey of Children and
  Youth (NLSCY) is a long-term study of Canadian
  children that follows their development and
  well-being from birth to early adulthood. The
  NLSCY began in 1994 and is jointly conducted by
  Statistics Canada and Human Resources Development
  Canada.

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NLSCY

• There are 4 cycles
• There are 8 different files
• 2 of these are available as a PUMF
• Primary
• Self-Reporting
• The rest include secondary and those based on
  people reporting about child (teacher,
  principal)

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Types of Microdata Files

• Confidential Microdata Products
• Master Files
• Share Files
• Public Access Microdata Products
• Public use anonymized microdata (PUMFS)
• Synthetic Files

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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

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Microdata Products

• NLSCY cycle 3 - primary

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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.

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Confidential Microdata

• Master File Example

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Confidential Microdata

• Master File detailed identifiers

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Confidential Microdata

• Master File geography

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Confidential Microdata

• Master File - fullness of data

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Confidential Microdata

• Master File - fullness of data

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Confidential Microdata

• Master File - fullness of data

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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

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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

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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

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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

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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.

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Public Access Microdata

• PUMFs personal identifiers

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Public Access Microdata

• PUMFs gross geography

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Public Access Microdata

• PUMFs collapsed data

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Public Access Microdata

• PUMFs suppressed variables
• Note from the MASTER file NOT the PUMF

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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

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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

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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.

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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

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Public Access Microdata

• Synthetic Files NLSCY

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Public Access Microdata

• Synthetic Files NPHS 1999 General File

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Implications for Analysis

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

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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

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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

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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).

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Implications for Analysis

• Public Use Microdata (PUMF)
• Valuable content for a tremendous amount of
  research
• Suppresed observations
• Suppressed variables
• Suppresed Content
• Gross Geography
• Collapsed categories
• Capped variables
• 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

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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)

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Implications for Analysis

• Synthetic Files
• Looks like a duck and quacks like a duck, but
  it isnt a duck or any other type of fowl.

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Implications for Analysis

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

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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.

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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.

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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.

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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
• NLSCY, NPHS CCHS synthetic files on DLI FTP site

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Where do we get Access?

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

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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

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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

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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

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Which File is Appropriate?

• RDC

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Which File is Appropriate?

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

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Services for Synthetic Files

• 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 RDCs

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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.

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Services for Synthetic Files

• 1. Building SPSS and SAS system files for
  synthetic data
• The NLSCY synthetic data are distributed as a raw
  ASCII file with accompanying command files for
  SPSS and SAS
• Separate synthetic data files exist for each
  component of the NLSCY not all components have
  PUMFs

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Services for Synthetic Files

• 1. Building SPSS and SAS system files for
  synthetic data
• The synthetic data for the NLSCY cycle 3
  primary file, has 948 variables and 6,393
  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.

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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.

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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

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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 Teams
  to-do list
• mailtonlscy_at_statcan.ca

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Services for Synthetic Files

• 2. Information about Research Data Centres
• The collection of master files available through
  RDCs is listed on the STC website for RDCs
• Each RDC has its own website describing its
  services
• http//www.statcan.ca/english/rdc/index.htm

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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

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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

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Exercises