Participation Rates in Epidemiologic Studies
Introduction
There is little question that participation rates for epidemiologic studies have been declining over the past 30 years 1, 2, with even steeper declines seen in recent years (3). This declining study participation has been documented in studies that are conducted by academic researchers 3, 4, by governmental agencies (5), and by for-profit companies (6) alike.
An examination of changing participation rates in several well-established national surveys illustrates the trends in participation rates over the past few decades, although, as we shall discuss, there is substantial variability in participation rates between studies. The Behavioral Risk Factor Surveillance Survey (BRFSS) has been conducted by the Centers for Disease Control and Prevention (CDC) since 1984 (7). The BRFSS is a nationally representative survey aimed at documenting changes in population-level behavioral risk factors, health screening, and health care access. The overall decrease in BRFSS participation rate is well documented. The BRFSS median participation rate was 71.4% in 1993, 48.9% in 2000, and 51.1% in 2005, the year for which data are most recently available (8). Similar declines in participation rates have been reported by the Survey of Consumer Attitudes (SCA), which has been conducted since the 1950s by the Survey Research Center of the University of Michigan, with surveys taking place monthly since 1978 (9). The SCA is a random-digit-dial survey assessing consumer attitudes toward spending and expectations about the economy (10). The participation rate for the SCA was 72% in 1979, 60% in 1996, and 48% in 2003 (3). The National Comorbidity Survey (NCS) is the largest, and for many the “gold standard,” cross-sectional study in establishing prevalence of psychiatric disorders. The original NCS, conducted between September 1990 and February 1992, reported an 82.4% participation rate (11). By contrast, the NCS-Replication (NCS-R), conducted approximately a decade after the original survey, between February 2001 and April 2003, reported a participation rate of 70.9% (12). Other epidemiologic studies that report participation rates have also demonstrated a decreasing trend over past decades (13). For example, while the participation rate reported for the original assessment of the Framingham Heart Study, established in 1948, was 69% (14), the participation rate for the Multi-Ethnic Study of Atherosclerosis, initiated in 2000, was 59.8% (15).
Even studies that have reported relatively consistent participation rates over time have shown that refusals to participate have been increasing and that consistent participation rates have been maintained only at the expense of more extensive efforts to ensure participation of hard-to-reach persons who nonetheless do not refuse study participation (16). For example, although participation rates in the National Health Interview Survey (NHIS) remained consistently in the 95%–97% range from the 1960s through the 1980s, refusals made up a greater proportion of all nonrespondents during this time (2). Overall response rates to NHIS surveys have since decreased, with a response rate of 91.8% in 1997 (17) and 86.9% in 2004 (18). Similarly, although participation rates remained consistent between 85% and 97% between 1955 and 1993 (19), refusals have also been increasing for the Current Population Survey (CPS) (2), which is conducted by the US Census Bureau for the Bureau of Labor Statistics. Response rate for the General Social Survey decreased to about 70% in 2000, from steady rates between 74% and 82% between 1975 and 1998; the decreases were entirely due to an increase in refusals (3). The International Conference on Survey Nonresponse, held in 1999, convened panels of experts to discuss the roots and the implications of declining study participation rates and to bring greater attention to this growing problem (20), followed by the publication of one of the foremost texts on survey nonresponse (21).
This wholesale decrease in participation rate, or at the very least the increase in refusal, has, quite understandably, occasioned some concern among epidemiologists who have long considered a high study participation rate as one of the hallmarks of a “good” epidemiologic study. It is the purpose of this review to synthesize the issues that are central to epidemiologic thinking around declining study participation rates. We will consider what is meant by participation rates, the reasons why study participation has been declining, summarize what we know about who does participate in epidemiologic studies, and discuss the implications of declining participation rates. We conclude with a discussion of methods that may help improve study participation rates.
Section snippets
What is a Participation Rate?
A recent review of peer-reviewed studies published in 10 high-impact journals showed that a substantial number did not report information on study participation (13). This reluctance of investigators to report participation rates is perhaps understandable given the epidemiologic tendency to chide low participation rates as a sign of study inferiority. Perhaps even more prevalent in the epidemiologic literature is a tendency toward dissembling about what exactly is the “response rate” for a
Why has Nonparticipation been Increasing?
There are two central reasons why nonparticipation in scientific studies has been declining over the past 50 years: potential participants have been increasingly refusing to take part in scientific studies, and it has become harder to find persons who might be eligible study participants. We discuss each reason in turn.
Who Participates in Epidemiologic Studies Today?
Recognizing that study participation rates are declining, it is then of particular concern to epidemiologists planning studies to identify who is likely to participate in studies. Such awareness ahead of study implementation may help plan targeted recruitment.
Most systematic efforts to characterize who does, and who does not, participate in studies have focused on the demographic characteristics of study participants. There is clear evidence that women are more likely to participate in
Implications of Study Nonparticipation
Although, as noted earlier, study participation rates are often considered a cardinal feature of good epidemiologic study execution, in light of the growing challenges faced by researchers in obtaining high participation rates, it is worth revisiting the fundamental reasons for our concern with study participation rates. There are two key considerations in this regard.
The central concern about study nonparticipation is the issue of nonparticipation bias (or, as it is more commonly termed,
Improving Participation Rates in Epidemiologic Studies
Understanding the study characteristics that may modify the likelihood of study participation may help guide efforts aimed at improving study participation. Although the role of demographic characteristics, including gender, race/ethnicity, and socioeconomic position, has been discussed in the preceding paragraphs as determinants of differential study participation rates, there is very little evidence for measures that can be adopted in epidemiologic studies to specifically tackle demographic
Conclusion
In this review we have highlighted the forces shaping declining study participation rates. We suggest that these forces are, by and large, inexorable and out of the hands of epidemiologists, and that participation rates are likely to decline further in coming decades. In the face of such bad news, the good news is that most empiric work suggests that declines in participation rates are not likely to have substantial influence on exposure-disease associations or point estimates of measures of
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