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Working Paper Number SEHSD-WP1989-12 or SIPP-WP-87
Karen E. Smith
Component ID: #ti1578498926


Analysts who want to examine events that occur over time face the problem of how best to process information from longitudinal data files. The standard approach for processing such data is to create a "person-linked" file, by linking a11 periods of information for each person onto one record. Person-level longitudinal analysis involves looking at each person-record to see how characteristics  or behavior changed over time. While this approach works if only the individual's characteristics are  of interest, it is cumbersome if the analyst wants to look at groups of people such as families or households, because such groups may change over time – people move out of households, people marry into families, people die.

An alternate approach for processing longitudinal data is to create a "person-month" file, by keeping each period of information for each person on its own record. Person-level longitudinal analysis involves looking at multiple records for each person to see how characteristics or behavior changed over time; looking at multiple records across months is equivalent to looking at a single person-linked record in the other approach. This paper will demonstrate that the person-month file is less cumbersome than the person-linked file for analysis of groups of people. This approach also provides substantial resource savings, even if only person-level analysis is done.

This paper illustrates the use of both person-linked files and person-month files for doing longitudinal analysis with monthly data from the Survey of Income and Program Participation (SIPP). First, some background pertaining to the SIPP and general problems of defining units is provided. The paper then describes and examines each method's ability to do person-level analysis and unit-level analysis. In this instance, all examples of unit-level analysis use the family as the unit, although the results are equally applicable to other types of units. It then compares the two methods of processing and their resource requirements. Finally, it includes an appendix with computer code in both PL1 and SAS that implement the person-month method.

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