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January 28, 1999

MEMORANDUM FOR         Daniel H. Weinberg
                                               Chief, Housing and Household Economic Statistics Division

From:                                      Donald J. Hernandez
                                               Special Assistant

Subject:                                  Comparing Response Rates for SPD, PSID, and NLSY

Introduction

The usefulness of data from any longitudinal or panel study that follows and interviews the same
respondents over a period of years depends on the assumption that the data represent the relevant
populations.  Nonresponse by members of the sample is a potential source of bias that can
undermine the quality of estimates derived using longitudinal data.  This memorandum compares
overall response rates between the initial interview and the most recent interview for three
major national surveys, the Survey of Program Dynamics (SPD) conducted by the U.S. Bureau of the Census, the Panel Study of Income Dynamics (PSID) conducted by the Survey Research Center at the University of Michigan, and the National Longitudinal Survey of Youth (NLSY) conducted by the Center for Human Resource Research at Ohio State University.  It also discusses major differences across the surveys in the approaches used to maximize long-run response rates, and the consequences of differential nonresponse for data quality.

Overall Response Rates and Alternative Approaches to Maximizing these Rates

Table 1 presents the current overall response rates for specified survey periods, both including
and excluding deceased persons from the base (These are the most recent, final estimates
published or available for each survey).  The current mortality-adjusted cumulative response rate for the entire survey period between initial sample selection and the most recent interview is 50 percent for the SPD, a level substantially less than the rate of 64 percent for the NLSY, but
substantially higher than the rate of 39 percent for the PSID (See two bottom lines of Table 1).
Among persons designated for interview during sample selection, these rates indicate the
proportion who were successfully interviewed during each round of interviews, that is, during
each interview period, without a single period of nonresponse.

There are three strategies commonly employed to recover more data in longitudinal studies: (1)
paying monetary fees to respondents, (2) returning to respondents who declined to be interviewed during a previous interview period, and (3) augmenting the second strategy by recovering data missed due to nonresponse during earlier interview periods.  For a longitudinal or panel survey spanning a substantial number of interviews, it is not essential that persons be interviewed in each and every interview period to provide valuable data for an important genre of analyses, two-point-in-time before-and-after analyses that are likely to be among the most prominent applications of SPD data in assessing the effects of welfare reform.  In fact, sample cases can provide valuable information even if interviews are separated by several years.  For these and other reasons, the NLSY has always sought to maximize its long-term response rates by efforts to recontact respondents lost from the sample and return them to the status of interviewed
respondents one or more years after an original nonresponse.  These efforts are extremely
successful.  For example, excluding the deceased from the base, the NLSY response rate of 71.5
percent between the first and most recent interview (line 3 of Table 1) jumps to 83.0 percent
when it is calculated to include persons who participated in 16 of the 17 NLSY interviews, and it jumps to 87.6 percent (nearly 90 percent) when it is calculated to include persons who
participated in 15 of the 17 NLSY interviews!  WOW!  Drawing on the NLSY experience, the
PSID recently began to experiment with and implement efforts to recontact respondents lost from
the sample and return them to the status of interviewed one or more years after their initial
nonresponse.  Without pursuing these efforts as vigorously as it might have (because of limited
funds), the PSID increased its effective sample size by 9 percent in a single year, increasing the
response rate for persons interviewed in interview 1 and in the most recent interview from 54 to
59 percent.

The NLSY and the PSID also use monetary incentives to encourage responses from interviewees.  Many studies have shown that response rates are improved substantially by monetary incentives.

Overall, at the present time, the SPD response rate is comparable to the other two major national
longitudinal surveys focused on economic and family behavior, but the SPD does not currently
employ any of these three NLYS and/or PSID strategies to maximize response.  The potential
problem that this poses for the SPD during the next few years is the subject of the next section of
this memorandum.

Future Response Rates in the SPD

Although the current cumulative response rate for the SPD lies between the corresponding rates
for the PSID and NLSY, the latter surveys have been in the field much longer (27 years and 17
years, respectively, compared to 6 to 7 years for the SPD), and they have involved larger numbers of field administrations (27 and 17 interviews, respectively, compared to 9 to 12 for the SPD). Many PSID and NLSY interviews occurred, however, in the 1970s and 1980s when response rates were generally higher for all surveys than has been the case in the 1990s.   In the SPD, the response rate between the last SIPP interview (the 1992 and 1993 panels) and the first SPD interview (the 1997 SPD bridge) was only 79 percent, and preliminary estimates suggest that the response rate between the 1997 SPD bridge interview and the 1998 SPD interview was about 83.4 percent overall, or 85.2 percent excluding both deaths and the newly-institutionalized
population from the denominator, for a preliminary estimate of a full panel SPD response rate of
48.7 percent, or an official household-based estimate of 50.0 percent (presented in Table 1).  If
the response rate remains at about 83 to 85 percent in subsequent annual interviews between
1999 and 2001 or 2002, the total cumulative response rate would fall to about 27 to 30 percent in 2001 and 23 to 26 percent in 2002, depending on whether or not the deceased and
newly-institutionalized populations are included in the base.  A response rate of 23 to 26 percent would be markedly less than the current directly-comparable rates of 39 percent for the PSID and 64 percent for the NLSY.  Although as suggested in the following section, estimates from such SPD data might be unbiased, concern about the quality of SPD estimates would surely increase.

Nonresponse and the Quality of Estimates

Response and attrition over time tends to be highly selective of persons who are concentrated
among lower socioeconomic status individuals.  This has been shown in analyses of the PSID
(Fitzgerald, Gottschalk, and Moffit: 1997a, 1997b; van den Berg and Lindeboom, 1997; Zabel,
1997; Ziliak and Kniesner, 1979) the NLSY (Falaris and Peters, 1997; MaCurdy, Mroz, and
Gritz, 1997), and the SPD and SIPP (Lamas, Tin, and Eargle, 1994; Tin, 1996; Zabel, 1997).
These analyses did not, however, find evidence that sample nonresponse or attrition leads to
seriously distorted estimates pertaining to a variety of topics.  Although the research addressing
this issue is much more extensive for the PSID and the NLSY than for the SPD and the SIPP, the
similarities across surveys in the nature of nonresponse and attrition suggest that estimates from
the SPD are in general likely to be no more biased than estimates from the PSID and NLSY; that
is, they are likely to be of acceptably high quality.  Nevertheless, in view of the fact that the
primary purpose of the SPD is to evaluate the effect of welfare reform among the low
socioeconomic status population, if the SPD does not improve its year-to-year response rates,
particularly among persons of low socioeconomic status, concerns about the quality of SPD
estimates will surely increase during the next few years.

Table 1.  Response Rates for SPD, PSID, and NLSY: Period-Specific and Total*
          (In percents)
                                                                                          PSID                      NLSY Interviews
Period-Specific                                         SPD     SRC    SEO    Total         Always   Currently
Sample-Selection to Interview 1                90.9     77       50.8     66.5           89.2            89.2
Interview 1 to Most Recent Interview
 All Deceased Included in Base                 51.6    45.2     45.2     45.2           69.6            86.7
 Known Deceased Removed from Base     53.6   53.0      53.0     53.0           71.5            NA
 Expected Deceased Removed from Base   NA     NA      NA       59              NA             NA
Cumulative
Sample-Selection to Most Recent Interview
 All Deceased Included in Base                  46.9     34.8      23.0   30.1             62.1            77.3
 Known Deceased Removed from Base      50.0     40.8      26.9   35.2             63.8            NA
 Expected Deceased Removed from Base    NA      NA       NA     39.2            NA             NA
-----------------------
*See attached notes for sources and detailed derivation procedures.  The estimated response rate
of 50.8 percent for the SEO portion of the PSID sample may be slightly too high, and my
historical investigation continues of the cumulative response rate between the point at which the
Census Bureau drew the original SEO sample and at which PSID interviewing began. "Known
Deceased" refers to persons specifically reported to interviewers as deceased in each survey.
"Expected Deceased" is calculated by PSID staff as of the 1994 interview by applying 5-year
life-table mortality rates to 1968 original sample individuals.  "Expected Deceased" has not been calculated for the SPD and NLSY, but if such estimates were calculated and the expected
deceased were removed from the base, the response rates for these two surveys would be slightly higher than estimates based on the known deceased.  The following provides a brief description for each survey of sample selection, interview 1, and the most recent interview.  NA means not available at present.

Survey of Program Dynamics -- Based on samples drawn for 1992 and 1993 SIPP panels
   Sample Selection --------- Samples drawn for 1992 and 1993 SIPP panels
   Interview 1----------------- Wave 1 interviews of 1992 and 1993 SIPP panels
   Most Recent Interview -- SPD interview in 1998, based on preliminary estimate for 1997-1998 response

Panel Study of Income Dynamics -- Based on samples drawn for the Census Bureau's Survey
of Economic Opportunity (SEO) and a special sample drawn by the University of Michigan
Survey Research Center (SRC) specifically for the PSID
   Sample Selection --------- Samples drawn for 1966/1967 SEO and 1968 PSID
   Interview 1----------------- Wave 1 interviews of PSID combined sample in 1968
   Most Recent Interview -- Wave 26 PSID interviews in 1993, "Expected deceased" is for 1994

National Longitudinal Survey of Youth -- Based on sample drawn for NLSY
   Sample Selection --------- Sample drawn for 1979 NLSY
   Interview 1----------------- Wave 1 interviews of NLSY in 1979
   Most Recent Interview -- Wave 17 interviews of NLSY in 1996

NOTES TO TABLE 1:
SPD -- Household response rates between sample-selection and the first interview were
calculated for the SIPP 1992 and 1993 panels combined, based on results presented in U.S.
Bureau of the Census internal memoranda for The Record from Michael F. McMahon on
November 27, 1995 for Final 1992 Panel Type A and Type D Sample Loss Report and Final
1993 Panel Type A and Type D Sample Loss Report.

Persons interviewed in the first nine waves of the SIPP 1992 and 1993 samples became the SPD
sample.  Individual response rates between the first SIPP 1992 and 1993 panel interviews and the SPD "bridge" interview (1997) were derived by Donald J. Hernandez using the SIPP 1992 Panel Waves 1-10 Longitudinal File, the SIPP 1993 Panel Waves 1-7 Longitudinal File, and the U.S.

Bureau of the Census internal SPD 1997 file available on the Housing and Household Economic
Statistics Division server on December 4, 1998.  Deceased are identified from the SIPP data for
the period between interview 1 and the final SIPP interview prior to the SPD interview.  The
response rate between SPD 1997 and SPD 1998 is preliminary, and removed from the base was
both deceased and newly institutionalized populations.  The estimate of 48.7 percent for the
cumulative person-based estimate with known deceased removed from the base is slightly less
than the estimate of 50.0 percent officially derived by the Bureau of the Census at the household
level.  The estimate of 50.0 percent is presented in Table 1.

PSID -- The PSID user's guide (Hill, 1992) notes that the original PSID sample actually
consisted of two independent samples, one drawn by the Survey Research Center and referred to
as the SRC sample, the other selected from the Survey of Economic Opportunity (SEO)
conducted in 1966 and 1967 by the Bureau of the Census for the Office of Economic Opportunity
(OEO).  The initial response rate for the SEO sample (Survey Research Center, 1972)  was
calculated to be 74 percent, based on the sample of households provided to SRC by the Bureau of the Census and the Office of Economic Opportunity.  This does not include the effect of (1)
attrition between sample selection by the Census Bureau and the first interview by the Census
Bureau of respondents in 1967, which led to a response rate was 91.6 percent (Office of
Economic Opportunity, 1970), (2) sample loss through subsequent refusals to remain in the
sample that became the SEO component of the PSID, since about 25 percent refused to allow
their names to be passed to SRC (Hill 1992), and (3) the failure of some sampled addresses to be transmitted from OEO to SRC (Hill, 1992).  To calculate the PSID SEO sample-selection to
interview response rate of 50.8 percent, I multiplied the initial response rate of 91.6 percent, by
the 75 percent rate of willingness to have names transmitted from the Bureau of the Census to
SRC, by the 74 percent response rate obtained by SRC in seeking to interview households
provided by the Bureau of the Census and the OEO.  This does not take into account that address
information for some sample persons who were willing to participate was not transmitted from
the Bureau of the Census and OEO to SRC.  I am conducting historical research to ascertain the
magnitude of this sample loss.  Introducing this source of sample loss into the calculations will
reduce the current estimate of 50.8 percent.  Of course, as in all the surveys discussed here,
weighting procedures were designed to take various factors, including sample attrition, into
account.  The response rate for the SRC sample was 76 percent.  The SRC sample constituted
about 60 percent of the initial PSID sample, while the SEO sample constituted about 40 percent
of the initial PSID sample.

The response rates for "interview 1 to the most recent interview" were obtained Survey Research Center (1972) and from Table 2a of the documentation provided by PSID via internet and titled "A Panel Study of Income Dynamics: Procedures and Code Books, Guide to the 1993
Interviewing Year, Procedures, Wave XXVI, A Supplement" obtained December 1998.  The first and most recent interview years were 1968 and 1993.  Techla Loup of the PSID was very helpful in identifying needed estimates and confirming the interpretation of specific estimates.  Deceased are identified by PSID staff from the PSID data between the first and most recent interviews.

Sandra Hofferth provided the estimated response rate for 1994 with expected deceased removed
from the base.

NLSY --   The source for these estimates is "NLSY79 Users' Guide, A Guide to the 1979-1996
National Longitudinal Survey of Youth Data" prepared for the U.S. Department of Labor by the
Center for Human Resources Research, the Ohio State University, August 1997.  The response
rate of 89.2 percent between sample selection and first interview is obtained from Table 3.3.1
based on the cross-sectional and supplemental (sub)samples.  The response rate of 69.6%
between the initial interview (1979) and the most recent interview (1996) is obtained from Table 3.7.1.  Deceased who number 224 by 1994 according to Table 3.6.1 were removed from the base. An additional 39 deaths (personal communication, Randall J. Olsen) for years 1995 and 1996 were also removed from the base. The "always" interviewed column includes in the numerator those persons who were interviewed in each of the 17 interviews.  The "currently" interviewed column includes in the numerator persons who were interviewed in at least the first interview and the current interview.  If the base is limited to those not dropped from the survey or deceased, the proportion of those interviewed in the first interview who missed no more than one interview out of 17 was 83.0 percent, and combined with the 4.6 percent who missed only two out of 17 interviews, the response rate was 87.6 percent.

Technical Note:  Accounting for "Expected" Mortality

Although each of the response rates in Table 1 for the period between sample-selection and
interview 1 would be slightly higher if mortality were taken into account by removing deceased
from the base, and although the response rates for the period between interview 1 and the most
recent interview for the SPD and NLSY would be slightly higher if "expected" mortality beyond
that reported to interviewers were taken into account as it is in the corresponding PSID response
rate of 59 percent, the general pattern of differences across the surveys would be unchanged for
rates that were fully adjusted for mortality.

                            REFERENCES

Center for Human Resource Research, Ohio State University. 1997.
     NLSY79 Users' Guide: A Guide to the 1979-1996 National Longitudinal Survey of Youth
     Data. Columbus, OH: Center for Human Resources Research.

Hill, Martha, 1992.
     The Panel Study of Income Dynamics: A User's Guide. Newbury CA: Sage Publications,
     Inc.

Fitzgerald, John, Peter Gottschalk, and Robert Moffit. 1997a.
     An Analysis of Sample Attrition in Panel Data: The Michigan Panel Study of Income
     Dynamics. The Journal of Human Resources XXXIII: 251-299.

Fitzgerald, John; Peter Gottschalk, and Robert Moffit. 1997a.
     An Analysis of the Impact of Sample attrition on the Second Generation of Respondents
     in the Michigan Panel Study of Income Dynamics. The Journal of Human Resources
     XXXIII: 300-344.

Lamas, Enrique, Jan. Tin, and Judith Eargle. 1994.
     The Effect of Attrition on Income and Poverty Estimates from the survey of Income and
     Program Participation (SIPP), U.S. Bureau of the Census Working, SIPP Working Paper
     9402.

Lillard, Lee A. And Constatijn W.A. Panis. 1979.
     Panel Attrition from the Panel Study of Income Dynamics: Household Income, Marital
     Status, and Mortality. The Journal of Human Resources XXXIII: 437-457.

MaCurdy, Thomas; Thomas Mroz, and R. Mark Gritz. 1979.
     An Evaluation of the National Longitudinal Survey on Youth. The Journal of Human
     Resources XXXIII: 346-436.

Office of Economic Opportunity, 1970
     A comparison of Selected Economic and Demographic Characteristics from the 1966 and
     1967 Surveys of Economic Opportunity and the Comparable Current Population Survey.
     Transmitted on January 20, 1970, by Carol B. Kalish, Office of Economic Opportunity,
     Executive Office of the President to Distribution List of Persons at Bureau of the Census,
     Bureau of Labor Statistics, Department of Commerce, Department of Health, Education
     and Welfare, Brookings Institution, Pennsylvania State University, and Urban Institute.

Survey Research Center, University of Michigan, 1972.
     A Panel Study of Income Dynamics: Study Design, Procedures, Available Data, 1968-1972
    Interviewing Years (Waves I-V), Volume 1.  Ann Arbor, MI: Institute for Social  Research.

Survey Research Center, University of Michigan. 1998
     A Panel Study of Income Dynamics: Procedures and Codebooks (Documentation), Guide
     to the 1993 Interviewing Year, Procedures, Wave XXVI, A Supplement.  Ann Arbor, MI,
     Institute for Social Research.

van den Berg, Gerard J. And Maarten Lindeboom. 1979.
     Attrition in Panel Survey Data and the Estimation of Multi-State labor Market Models.
     The Journal of Human Resources XXXIII: 458-478.

Ziliak, James P. and Thomas J. Kniesner. 1997.
     The Importance of sample Attrition in Life Cycle Labor Supply Estimation. The Journal
     of Human Resources XXXIII: 507-530.

Zabel, Jeffrey E. 1997
     An Analysis of Attrition in the Panel Study of Income Dynamics and the Survey of
     Income and Program Participation with an Application to a Model of Labor Market
     Behavior. The Journal of Human Resources XXXIII: 479-506.

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