Managing and Analyzing Longitudinal Data COPAFS Quarterly Meeting June 1, 2012

1. Managing and Analyzing Longitudinal DataCOPAFS Quarterly MeetingJune 1, 2012 Patricia RugglesCatherine Ruggles patricia@orlinresearch.comcatherine@orlinresearch.com 240-350-6457 213-324-4234

2. Longitudinal Data are Hard to Use • Longitudinal databases tend to be very complex • Complex documentation and record linkage issues: searching and understanding variable lists, record structures, and other features requires patience and persistence • Creating analysis files typically involves major data restructuring • Files are often hierarchical as well as linked across time periods; variables need to be moved across record types, new variables need to be created involving more than one record type, etc. • Longitudinal analyses involve complex relationships across records and variables and therefore can be conceptually difficult to plan and carry out

3. Results: Under-use and Misuse • Analysts shy away from using large longitudinal data sets such as SIPP because understanding and restructuring the data is frustrating, expensive and time-consuming • When such datasets are used it is often for cross-sectional rather than longitudinal analyses—e.g., topical modules in SIPP—or to compare two points in time, rather than to examine patterns of activity over time • As a result: under-use, funding difficulties, low return on our investment in data collection and preparation

4. Longitudinal Analysis Steps • Step 1: Understanding the Data • Explore metadata and data and choose appropriate variables • Step 2: Preparing Data for Analysis • Recode and create variables as necessary • Step 3: Performing Analyses • Perform cross-sectional and longitudinal analyses as desired

5. Step 1: Understanding the Data • Many longitudinal datasets are very large and not necessarily well documented • For example: The 2008 SIPP has 48 months of data on just under 120,000 unique individuals, and contains more than 1000 variables • Documentation exists in many places, but it can be hard to link specific variables to the appropriate questions in the questionnaire, and to understand issues such as the universe to which each variable applies • A key need for longitudinal data users, therefore, is a better way of exploring the available data and linking it to the appropriate metadata • Orlin has made the ability to search and understand both data and metadata a key feature of our system • Let’s do a quick tour of the data and metadata exploration system

6. The Welcome Page

7. Variable List for SIPP

8. Exploring SIPP Metadata and Data • To see the available variables, click on the person-month record type in the metadata tab on the Welcome Page • There are over 1000 variables—one of the things that makes SIPP hard to use! • To find a specific variable, type its name or any other identifying information in the search box • This brings up all variables meeting the search criteria— • e.g., typing employment will bring up the 39 variables relating to employment, along with their labels and codes • To select a specific variable, click on it • Will show its codes, frequencies, and summary statistics • Also, hyperlinks to related variables and to all citations for this variable in questionnaires, code books, and user guide

9. Variable Search Results: Employment Status Recode Variable

10. Viewing the Data • In addition to hyperlinks to other metadata, the metadata are linked directly to the data • For example—clicking on the number of cases with a specific code value in the frequency table will bring up all the case records with that value • Users can choose which variables on those records they wish to inspect, using a drop down check list • This aids in debugging, understanding complex variable recodes

11. Finding the Information You Need • The search and hyper-linking features of the Orlin System address the first of the difficulties in working with SIPP discussed earlier in our presentation • Many users give up before they even get to longitudinal analysis, because it can be so hard to find the right variable and its associated documentation • SIPP documentation is still a bit patchy, but by hyper-linking all existing documentation for every variable the Orlin System makes it much easier to understand exactly what the variable means • The system also includes a global search function, which allows users to search across all aspects of the system for any specific phrase or term

12. Step 2: Preparing Data for Analysis • Longitudinal data require substantial manipulation and recoding before analysis, even after finding the right variables • Creating usable data extracts that preserve necessary information on relationships between units of analysis and their individual components can be complex even in cross-sectional data • Adding a time dimension means moving information across both record types and points in time • Sample attrition, the addition of special supplements, inconsistencies in responses across waves of the survey, and weighting problems pose additional difficulties • Users need help in understanding and dealing with these issues

13. The Longitudinal Unit of Analysis • Longitudinal Surveys such as SIPP, the Health and Retirement Survey, etc. typically contain data on several potential units of analysis or record types, such as households, persons, welfare units, medical records, etc. • For most types of longitudinal analysis, only units that are unchanging over time can be usefully linked across time • For example— can’t link households over time because they change too much from period to period • For most demographic surveys the person-month (or person-year) record is the basic longitudinal unit—simply a string of linked records across time for each person • Information from associated units or record types must then be linked to the longitudinal unit at the appropriate point in time

14. Restructuring Longitudinal Data • Creating the necessary links is very difficult using sequential data processing packages such as SAS • The process will require several steps, each of which means a new pass through the data set • For example, to track each person’s household income in each month of the survey using SAS: • 1. Find the correct household for person 1 this month • 2. Create a summary variable for household income that month • 3. Attach that variable to the person-record for that month • 4. Repeat for next month for person 1 • 5. After creating household income variables for each month for person one, repeat for persons 2 – 50,000 • This gets old fast, especially because it has to be repeated for many variables—age of head, welfare recipiency—and for many record types—subfamilies, welfare units, etc.

15. The Orlin Approach to Restructuring Data • The Orlin system uses database technology to keep track of variables and their linkages across both record types and time • This greatly simplifies the process of transforming variables as needed, creating new variables, and making sure that all variables are useable appropriately in longitudinal analyses • This also simplifies the process of recoding variables and performing other data transformations that are typically needed in both cross-sectional and longitudinal analyses

16. SIPP Data Structure in the Orlin System • We will use the 2008 SIPP panel to illustrate how the Orlin restructuring system works. • The basic record type is the person-month record, which is the series of all of the months of data for a specific person. • We have also created records for each unique person, family or household that ever appears in the panel. • Records are stored in a database system that understands their linkages, which makes it easy to create variables that draw on data from different record types or different points in time.

17. Preparing Data for Analysis • Finding the right variables is only the first step • Even in cross-sectional analyses, variables may need to be recoded for a specific analysis—for example, by collapsing the number of codes • Sometimes new variables need to be created by combining information from two or more existing variables—for example, using income and family size to calculate equivalent income across different families • Sometimes information on other people must be used in conjunction with variables on the person-month record—for example, to identify workers with pre-school children • All of these examples require data transformations and the creation of new variables

18. Data Transformations • The Orlin System allows intelligent data transformations because records are linked internally in a database, and the system understands those links • Transformations such as recodes and the calculation of new variables require two steps in the Orlin System: • First, the new variable is defined, using the system’s templates • Second, when a satisfactory definition has been created, it is run on the data to actually create the variable • New variables can be created using either a small sample of about 35,000 person-month records, or the full sample, which includes about 2.6 million records. • The small sample runs in the foreground and takes up to 5 mins. • The full sample runs in the background and takes considerably longer, depending on the complexity of the transformation.

19. Creating a New Variable Definition • The first step in transforming data is to define the new variable you want to create • Second step: Run the new definition on the data to create the new variable • Orlin automatically tracks every change, every new definition, and all output

20. Template for Variable Definition

21. Example: Run Variable Creation for ANY_WORK

22. Audit Trail

23. Complex Transformations • A particular strength of the Orlin System is its ability to handle complex data transformations, such as creating variables that use data from different record types and/or different months • Example: creating AVERAGE_EARNINGS for an individual across all months of the panel • Create new variable definition as before, specifying new variable name and source variable (TPEARN) • Select create a complex variable • Select “average” under function type • Select sample and run

24. Example: Complex Data Transformations

25. Step 3: Performing Analyses • After transforming our data as needed, we are ready to analyze them • To analyze data using the Orlin System, press the Analyze button on the home page button bar • Specific analyses such as crosstabs, regressions, and duration analyses can be performed by clicking on the appropriate button • A template will appear asking for the information needed for the requested analysis: for example, for a regression, the type of regression, the dependent variable, and the independent variables • Analyses use the R statistical system • Results of data transformations can also be exported for analysis in statistical packages such as SAS, SPSS and Stata

26. Example: Regression Results

27. Longitudinal Analyses • In addition to standard cross-sectional analyses, the Orlin System allows various types of time-related analyses • In particular, it can perform two main types of longitudinal analysis: • Analysis of transitions—changes in state such as moving from employment to unemployment—and the relationship of such changes to other variables or other changes • Analysis of spells—periods of time over which a changed state persists, such as a spell of unemployment—and the effects of other variables on the duration of such spells

28. Defining Transition Variables • Clicking on the Create Transition Variable button in the transform area brings up a template that allows the user to define the specific state change of interest • Example: STOP_WORK • This variable is defined as a change from the status of working to the status of not working • It uses the ANY_WORK variable we previously defined • The user can choose to identify either in the last month worked or the first month not working, by choosing to compare to the previous or following month • The variable uses the time variable SEQUENCE, which is simply the sequence number of the month (eg, 32 for the 32nd month in the panel)

29. Create a Transition Variable: STOP_WORK

30. Defining Spells • A spell is a period of time defined by two transitions—into the state of interest (such as unemployment), and out of the state • A spell may occur even if only one transition is observed —if for example someone becomes unemployed but the panel ends before the unemployment spell does • Such as spell would be right-censored—no ending can be observed • Spells can also be left-censored—an ending is observed, but no beginning • Statistical techniques exist to analyze spells durations, accounting for censoring

31. Duration Analysis • Standard duration analyses essentially calculate the proportion of all those observed in a spell at a given point in time who exit the spell at that point—in other words, the “hazard” of leaving the spell • Analyses can take into account the effects of various independent variables on predicted durations • The Orlin System allows a variety of different models to be explored • All of these duration models operate on the spell record

32. Create a Spell Record

33. Spell Record Variables

34. Example: Spell Records

35. Analyzing Spell Records • The basic spell record includes only basics relating to the spell itself • To analyze durations in conjunction with anything else, therefore, the independent variables of interest have to be moved to the spell record • This can be done using the create variable definition screen, choosing the option to move a variable

36. Duration Analysis

37. Duration Analysis: Results

38. Conclusion • Analyzing longitudinal datasets requires three steps: • Finding the appropriate information • Restructuring it for longitudinal analysis • Performing the analysis and examining the results • All of these are hard to do using analysis packages such as SAS, Stata or SPSS • The goal of the Orlin System is to simplify all three steps • We link and provide search capabilities across data and metadata • We use database technology to keep track of both data and metadata, cross-sectionally and over time • We provide easy-to-use templates to guide the analyst through the entire process • If you are interested in learning more or becoming a beta user, see our website, www.orlinresearch.com, or contact us

39. Thank You! www.orlinresearch.com