Self-selection bias in an epidemiological study of respiratory health of a rural population

A common source of bias in epidemiological studies is self-selection or volunteer bias. The self-selection into cohorts for further investigation or for participation in a substudy may be an important source of selection bias. The objectives of this paper were to identify differences in selected demographic characteristics, individual and contextual factors, and variables related to respiratory health between study participants who were willing to participate in subsequent clinical and allergy testing and those who were not willing to participate. One hundred and fourteen households (207 individuals) from a small town and 54 households (99 individuals) from a nearby rural municipality participated in this study. One key informant from each household provided information about each adult in that household. The question being studied was: "We wish to find out more about respiratory health of rural people. We would like to invite you to perform breathing and allergy tests. Would you be willing to be contacted for breathing and allergy tests in a nearby location?" One hundred and four participants said "Yes" they would participate in the clinical studies, 144 said "No" they would not participate in the clinical studies, and 52 said "Would like more information" about the clinical studies. More than one half (53.8%) of male participants and 46.2% of female participants indicated that they would like to participate in breathing and allergy tests. A higher proportion of study participants (26.1% males, 30.8% females) in the lowest income category requested more information compared to those answering either "Yes" (15.7% for males, 20.5% for females) or "No" (18.5% males, 23.3% females) to the question being studied. Study participants who were willing to participate in further breathing and allergy tests had a higher proportion of self-reported chronic phlegm and ever had allergic reaction to things eaten than those who either said "No" or "Would like more information." Among male study participants who said "Yes" to further participation, a higher proportion was exposed to one of the occupational exposures of interest compared to those who said either "No" or "Would like more information." This pattern was not observed for females.     

One Key Question® and the Desire to Avoid Pregnancy Scale: A comparison of two approaches to asking about pregnancy preferences

ObjectiveTo facilitate assessment of patients’ pregnancy preferences, we compared responses to One Key Question® with the validated Desire to Avoid Pregnancy (DAP) scale and assessed their relationships to patient-reported reproductive health behaviors.MethodsIn this after-visit survey in primary care and obstetrics-gynecology practices, women ages 18–49 (n = 177) answered “Would you like to become pregnant in the next year?” and the 14-item DAP scale. We performed one-way ANOVA to compare DAP scores (0–4 scale, 4 = highest preference to avoid pregnancy) across One Key Question® responses (“Yes,” “Unsure,” “Ok either way,” “No but sometime in the future,” “No never”). We used logistic regression to test association of One Key Question® and DAP with contraceptive and folic acid use.ResultsMost patients did not want to become pregnant in the next year, based on One Key Question® (7% “Yes,” 4% “Unsure,” 11% “Ok either way,” 53% “In the future,” 25% “Never”). The mean DAP score overall was 2.52 (SD = 1.03, Range: 0–4, Cronbach’s α = 0.96). Scores differed by One Key Question® response (“Yes” mean DAP = 0.84, “Unsure” 1.64, “Ok” 1.42, “In the future” 2.94, “Never” 2.78, p < 0.001) yet varied markedly within each One Key Question® response group. Contraceptive use was lower among those who answered “Yes” (46%; OR = 0.14, 95% CI 0.04–0.48) vs. “No, future” (86%). Similarly, odds of contraceptive use increased with DAP score (OR = 1.69, 9% CI 1.18–2.42; predicted 51% for DAP = 0, 90% for DAP = 4).ConclusionOne Key Question® responses correlate with DAP scores, and contraceptive use correlates with not desiring pregnancy by both approaches.Implications StatementOne Key Question® and the Desire to Avoid Pregnancy scale can both identify women wishing to avoid pregnancy to help clinicians address patients’ contraceptive needs. Given the range of preferences associated with One Key Question® responses, clinicians who use it should proceed with further discussion to fully understand patients’ feelings.     

Oral-Genital Contact and the Meaning of “Had Sex”: The Role of Social Desirability

Previous studies have found that a large proportion of college students do not consider oral-genital contact as having “had sex.” In all studies, the questions posed were hypothetical. In the present study, university students were asked about their own personal sexual experiences. From a large pool of participants, two subgroups were identified: those who responded “No” to having “had sex” but responded “Yes” to having had oral-genital contact (No–Yes), and those who responded “Yes” to having had both sex and oral-genital contact (Yes–Yes). None of the participants in these two subgroups self-reported vaginal or anal intercourse. The No–Yes group was significantly higher in social desirability (p?<?.0005) as measured by the Marlowe-Crowne scale and was also significantly higher in religiosity (p?<?.01) as measured on a 7-point Likert scale. There was a modest correlation between level of religiosity and social desirability (r?=?.25, p?<?.01). It was concluded that many students who have had oral-genital contact but deny having had sex do so because of impression management, i.e., a desire to present themselves more positively. These results provide further evidence that social desirability responding is a serious problem for sex researchers, one that affects even the most basic questions about having had sex.

     

Estudio de intervención formativa con miniensayos para mejorar la atención de las urgencias médicas en un centro de salud

ObjectiveTest effectiveness and acceptability of interventions short essay-type training in health emergency management (EM).DesignCombined case series and controlled study before and after training sessions.LocationHealth Center (HC).ParticipantsTeam on duty, two monitors-facilitators, and a mannequin.Main measuresVariables: response times, staff performance, resource usage and opinion. Structure: scenarios and key messages. Instrument development: 1. Initial/final questionnaire and events. 2. Essential/non-essential times; 3. Post-test opinion questionnaire. Performance of six consecutive 15 min tests fortnightly (including corrections) and poll after each test. A month later, repeat in random order and under similar conditions. Analysis: repeated measures.ResultsA total of 93 (2/3) workers completed the initial survey, and 74 the final, with 46 participants (25 doctors, 7 nurses, 21 non-health completed 95 direct interventions. Matching participants > 80% between series. A reduction was seen in the “detection of collapse to first defibrillation” interval (10 to 4 min). EM events improved 2-3 fold and “sense of security during a real EM” increased from 23% to 71% among participants. The vast majority of participants said “useful corrections made by the facilitator.” The proportions of those who “would like to see tests introduced” and those who said “re-training was needed in EM” were moderately increased (67.4% vs 85% in health care workers). The “would like to attempt basic life support” was unchanged.ConclusionDespite being reduced in number and duration, this model of intervention has shown positive trends in terms of use and acceptability for implementation in the HC.     

Trends,patterns and psychological influences on COVID-19 vaccination intention: Findings from a large prospective community cohort study in England and Wales (Virus Watch)

BackgroundVaccination intention is key to the success of any vaccination programme, alongside vaccine availability and access. Public intention to take a COVID-19 vaccine is high in England and Wales compared to other countries, but vaccination rate disparities between ethnic, social and age groups has led to concern.MethodsOnline survey of prospective household community cohort study participants across England and Wales (Virus Watch). Vaccination intention was measured by individual participant responses to ‘Would you accept a COVID-19 vaccine if offered?’, collected in December 2020 and February 2021. Responses to a 13-item questionnaire collected in January 2021 were analysed using factor analysis to investigate psychological influences on vaccination intention.ResultsSurvey response rate was 56% (20,785/36,998) in December 2020 and 53% (20,590/38,727) in February 2021, with 14,880 adults reporting across both time points. In December 2020, 1,469 (10%) participants responded ‘No’ or ‘Unsure’. Of these people, 1,266 (86%) changed their mind and responded ‘Yes’ or ‘Already had a COVID-19 vaccine’ by February 2021. Vaccination intention increased across all ethnic groups and levels of social deprivation. Age was most strongly associated with vaccination intention, with 16–24-year-olds more likely to respond “Unsure” or “No” versus “Yes” than 65–74-year-olds in December 2020 (OR: 4.63, 95 %CI: 3.42, 6.27 & OR 7.17 95 %CI: 4.26, 12.07 respectively) and February 2021 (OR: 27.92 95 %CI: 13.79, 56.51 & OR 17.16 95 %CI: 4.12, 71.55). The association between ethnicity and vaccination intention weakened, but did not disappear, over time. Both vaccine- and illness-related psychological factors were shown to influence vaccination intention.ConclusionsFour in five adults (86%) who were reluctant or intending to refuse a COVID-19 vaccine in December 2020 had changed their mind in February 2021 and planned to accept, or had already accepted, a vaccine.     

Public attitudes regarding willingness to participate in medical research studies

The recruitment of adequate numbers of people to participate in medical research studies is an ongoing problem for biomedical researchers. Although the general public has come to expect and demand that the biomedical community develop new, safe and effective approaches to the prevention and treatment of diseases, that same public is not aware of the important role that public participation plays in the development of medical advances. Much is known about willingness to participate in research studies from the perspectives of patients, survivors, and those at-risk for getting a particular disease. However, little is known about the attitudes and willingness of the general public to participate in medical research. Yet, it is this population that comprises the potential pool of participants for future treatment and prevention studies. In order to examine public attitudes toward and support for medical research, a random digit dial telephone survey was conducted with 489 persons in southwestern Pennsylvania. The survey measured the respondent's stated willingness to take part in a medical research study and the factors associated with willingness to participate. These included the respondent's health status, demographic characteristics, attitudes and beliefs about participation and their knowledge about the conduct of medical research. The results of the study indicate that 46% of those surveyed said that they would be willing to take part in a medical research study focusing on a new treatment for a specific disease that was of concern to them, 25% stated that they would not be willing and 29% stated that they were undecided regarding participation. However, under certain circumstances, such as having cancer, over half of those who were undecided said they would be willing to participate. The characteristics of those willing to participate in a medical research study differ from those not willing. Determinants of willingness include: having a relative or friend who has an illness, being middle aged (between 35-64 years old), prior experience with participation in a medical research study, having a favorable attitude toward the use of human subjects in medical research and beliefs that diverse types of persons participate in clinical trials. Those respondents who were undecided about joining a clinical trial, also have different characteristics than those who are not willing. The determinants of being undecided in contrast to not willing include: having at least a college degree, having a favorable or neutral attitude toward the use of humans in medical research and, believing that the well-being of participants is the primary concern of researchers. The findings of this study have both public policy and practice implications. From a policy perspective, medical research designed to develop new treatments for disease requires an evidenced-based approach for decision making. Such an approach can only succeed if adequate numbers of individuals are willing to participate in these studies. From a practice perspective, the current study suggests that opportunities exist to increase participation by targeting recruitment efforts not only toward the willing but also toward those who are undecided about participation in medical research studies. This would involve tailoring the content of communications to meet the specific characteristics and concerns of each of these two groups of individuals.     

Nutrition Beliefs and Weight Loss Practices of Lakota Indian Adults

Primary objectives were to describe beliefs about diet and health, weight perceptions, and weight loss practices among Lakota Indian adults. In-person interviews were conducted with a total of 219 adults from two reservations in South Dakota. Overall, 55.5% of the sample was overweight. When asked how they perceived their body weight, 6% felt they were “too thin,” 43.4% thought they were “about right,” and 50.2% felt they were “too fat.” When asked what they were doing about their weight, 74% said they were either trying to lose weight or trying to keep from gaining more weight. Reducing the amount of food eaten (90%), eating more fruits and vegetables (86%), increasing physical activity (84%), and skipping meals (82%) were reported to have been used by most of those who had dieted. Of those who felt they needed to lose weight, 78% said they would join a weight loss program if one were offered. Findings indicate that most Lakota adults are concerned about obesity and are attempting to either lose excess weight or avoid gaining weight. Intervention efforts for this population should focus on providing individuals with guidance on effective strategies for weight loss or obesity prevention. Weight loss programs should include features identified as important by survey participants, such as information on how to fit a healthful diet and exercise into daily life.     

Ethical issues associated with conducting genetic family studies of complex disease

PURPOSE: To examine subjects' recognition of the risks and ethical issues associated with enrollment in genetic family studies (GFS) and explore how this recognition affects their informed and voluntary participation. METHODS: A cross-sectional study design including both quantitative and qualitative data was employed. Structured interviews using the Contextual Assessment Approach Questionnaire (CAA-Q) were conducted with 246 Mexican American (MA) participants. To gain in-depth understanding of questionnaire responses, semi-structured interviews with 30 participants were conducted. All participants were interviewed before their enrollment in the Family Investigation of Nephropathy and Diabetes (FIND). RESULTS: Subjects' average age was 56 years; 62% were females. Seventy-eight percent of participants were not formally educated beyond high school and 72% reported an annual household income of < or =20,000 dollars. Eighty-five percent agreed to provide researchers with information on relatives' ages, gender, and education. Sixty-five percent of participants were willing to provide identifiable information such as names, addresses, and phone numbers of relatives. Sixty-three percent of participants indicated that there were direct benefits (i.e., supporting research) to disclosing relatives' information. Seventy-six percent stated that there were no risks associated with participation in GFS (e.g., discrimination or confidentiality of genetic information) compared with 10% who said that there were such risks. While discussing potential risks, subjects did not consider these to influence their decision to participate. CONCLUSIONS: Subjects enrolled in GFS did not recognize and tended to underestimate the social and cultural risks associated with their participation in GFS. If subjects do not fully comprehend the risks, this raises questions concerning their ability to provide informed consent and to voluntarily participate. We propose a subject-centered approach that views enrollment as an active process in which subjects and recruiters give and receive information on risks and ethical issues related to participation, which enhances protection of the rights and welfare of subjects participating in GFS.     

Effects of Solicitor Sex and Attractiveness on Receptivity to Sexual Offers: A Field Study

Gender differences in receptivity to sexual offers have been found in previous studies conducted in the United States. However, this effect has never been replicated in another culture, and the impact of the attractiveness of the solicitor remains in question. An experiment was conducted in France in which male and female confederates of average versus high attractiveness approached potential partners of the opposite sex (120 males and 120 females) and asked them: “Will you come to my apartment to have a drink?” or “Would you go to bed with me?” The great majority of the men were willing to have a sexual liaison with a woman, especially when she was physically attractive. Women were more disinclined to have a drink, and none but one accepted the male’s sexual request. Such results confirm that men are apparently more eager for sexual activity than women are.     

老年人参与“互联网+慢性病管理”意愿影响因素分析——基于Anderson健康行为模型的实证研究

目的:分析影响老年人参与“互联网+慢性病管理”意愿的因素。方法:基于Anderson健康行为模型,使用江苏、上海、浙江的调研数据,对老年人参与“互联网+慢性病管理”的意愿进行Logistic回归分析。结果:低龄老年人、女性老年人、城市老年人、不太了解慢性病知识的老年人、能独立使用智能设备的老年人、需要非药物治疗的老年人、参加体育锻炼的老年人,以及参加老年社团的老年人,更愿意参与“互联网+慢性病管理”。结论:从宣传、培训、资金投入、监督评估四个方面,提升“互联网+慢性病管理”的环境,促进老年人参与“互联网+慢性病管理”。     

Potential self-selection bias in a nested case-control study on indoor environmental factors and their association with asthma and allergic symptoms among pre-school children

Selection bias means a systematic difference between the characteristics of selected and non-selected individuals in epidemiological studies. Such bias may be introduced if participants select themselves for a study. The present study aims at identifying differences in family characteristics, including health, building characteristics of the home, and socioeconomic factors between participating and non-participating families in a nested case-control study on asthma and allergy among children. Information was collected in a baseline questionnaire to the parents of 14,077 children aged 1-6 years in a first step. In a second step 2,156 of the children were invited to participate in a case-control study. Of these, 198 cases and 202 controls were finally selected. For identifying potential selection bias, information concerning all invited families in the case-control study was obtained from the baseline questionnaire. Results show that there are several possible biases due to self-selection involved in an extensive study on the impact of the home environment on asthma and allergy among children. Factors associated with participating were high socioeconomic status of the family, more health problems in the case families, and health-related lifestyle factors, such as non-smoking parents. The overall conclusion of this study is that there are selection biases involved in studies that need close cooperation with the families involved. One solution to this problem is stratification, i.e. investigating associations between exposures and health in the same socioeconomic strata.     

Reducing Smoking in Pregnancy Among Māori Women: “Aunties” Perceptions and Willingness to Help

Māori (the indigenous people of New Zealand) women have high rates of smoking during pregnancy and 42 % register with a lead maternity carer (LMC) after their first trimester, delaying receipt of cessation support. We used a participatory approach with Māori community health workers (“Aunties”) to determine their willingness and perceived ability to find pregnant Māori smokers early in pregnancy and to provide cessation support. Three meetings were held in three different regions in New Zealand. The aunties believed they could find pregnant women in first trimester who were still smoking by using their networks, the ‘kumara-vine’ (sweet potato vine), tohu (signs/omens), their instinct and by looking for women in the age range most likely to get pregnant. The aunties were willing to provide cessation and other support but they said they would do it in a “Māori way” which depended on formed relationships and recognised roles within families. The aunties’ believed that their own past experiences with pregnancy and/or smoking would be advantageous when providing support. Aunties’ knowledge about existing proven cessation methods and services and knowledge about how to register with a LMC ranged from knowing very little to having years of experience working in the field. They were all supportive of receiving up-to-date information on how best to support pregnant women to stop smoking. Aunties in communities believe that they could find pregnant women who smoke and they are willing to help deliver cessation support. Our ongoing research will test the effectiveness of such an approach.     

Are young injection drug users ready and willing to participate in preventive HCV vaccine trials?

Trials to evaluate the efficacy of preventive HCV vaccines will need participation from high risk HCV seronegative injection drug users (IDUs). To guide trial planning, we assessed willingness of young IDU in San Francisco to participate in HCV vaccine efficacy trials and evaluate knowledge of vaccine trial concepts: placebo, randomization and blinding. During 2006 and 2007, a total of 67 participants completed the survey. A substantial proportion (88%) would definitely (44%) or probably (44%) be willing to participate in a randomized trial, but knowledge of vaccine trial concepts was low. Reported willingness to participate in an HCV vaccine trial decreased with increasing trial duration, with 67% of participants surveyed willing to participate in a trial of 1 year duration compared to 43% of participants willing to participate in a trial of 4 years duration. Willingness to enroll in HCV vaccine trials was higher in young IDU than reported by most at-risk populations in HIV vaccine trials. Educational strategies will be needed to ensure understanding of key concepts prior to implementing HCV vaccine trials.     

Women's opinion concerning oocyte donation for pre-embryonic studies

For the benefit of pre-embryonal research, women requesting sterilization may be asked whether they are prepared to donate egg cells. Recently, a questionnaire was sent to 144 women who had given birth to their second or third child at the University Hospital Groningen, asking them how they felt about egg cell donation: 68 (47%) of the questionnaires was returned. Of the respondents 45 (66%) felt that a request for donation could be made to women who were admitted to hospital for sterilization. 22 (32%) of the respondents said they would be willing to donate an egg cell themselves. Of these women, 10 said they would be willing to use hormone preparations for this purpose.     

Willingness of Chinese injection drug users to participate in HIV vaccine trials

BACKGROUND: Chinese injection drug users (IDUs) may be a proper candidate population for HIV vaccine trials. OBJECTIVE: To evaluate willingness to participate (WTP) in HIV vaccine trials among Chinese IDUs. METHODS: Questionnaire interviews were completed among 401 IDUs in Urumqi City in northwestern China in 2005. RESULTS: Overall 74.3% of participants said that they would be definitely willing to participate in HIV vaccine trials, 17.7% were probably willing, 6.2% were probably not willing, and remaining 1.8% were definitely not willing to join. Multivariate logistic regression analysis demonstrated that WTP was positively associated with having ever had sex with a drug use partner (adjusted odds ratio [AOR]: 1.8; 95% confidence interval [CI]: 1.04, 3.2), sharing needle and syringe with a new drug use partner in the past 3 months (AOR: 3.8; 95% CI: 1.2, 11.7), perceived family support for participation (AOR: 7.4; 95% CI: 4.3, 12.7), and perceived vaccine protection against HIV infection (AOR: 16.1; 95% CI: 3.7, 70.8), and was negatively associated with perceived risk of social stigma and isolation for participation (AOR: 0.3; 95% CI: 0.2, 0.5). CONCLUSIONS: The stated WTP in hypothetical HIV vaccine trials was high among Chinese IDUs. Further studies are needed to evaluate actual enrollment into the trials.     

The incidence of bulimia in freshman college students

This paper reports the results of a questionnaire survey administered to 1355 college freshman (98.3% response rate) and to a comparison group of 37 female bulimic patients. Questions were contructed to elicit information which would allow identification of those respondents who would meet DSM-III requirements for diagnosis of bulimia. 2.1% of the student population (4.5% of females, 0.4% of males) met these modified criteria and the additional criterion of weekly binge-eating. The identified “bulimic” female students differed from female bulimic patients in their use of fasting instead of self-induced vomiting for weight control, and in their tendency to be overweight. “Bulimic” female students are also compared with nonbulimic students.     

广西MSM人群定期应用HIV抗体口腔黏膜渗出液诊断试剂的意向及影响因素分析

目的了解男男性行为人群(men who have sex with men,MSM)定期应用人类免疫缺陷病毒(human immunodeficiency virus,HIV)抗体口腔黏膜渗出液检测试剂的意向及影响因素。方法采用方便抽样方法在广西招募MSM进行问卷调查,回收有效问卷283份。结果调查对象中89.0%(252/283)有意愿使用HIV抗体口腔黏膜渗出液诊断试剂,70.6%(178/252)愿意定期使用。多因素分析结果表明,最近六个月发生异性性行为(OR=0.247,P=0.009)、既往到疾病预防控制中心/医院检测(OR=0.183,P0.001)的MSM使用HIV抗体腔黏膜渗出液诊断试剂定期检测的意愿较低。结论广西MSM人群对HIV抗体口腔黏膜渗出液诊断试剂接受意愿较高,可以推广使用。     

Adolescent Males' Awareness of and Willingness to Try Electronic Cigarettes

PurposeElectronic cigarettes (e-cigarettes) are a new type of device that delivers vaporized nicotine without the tobacco combustion of regular cigarettes. We sought to understand awareness of and willingness to try e-cigarettes among adolescent males, a group that is at risk for smoking initiation and may use e-cigarettes as a “gateway” to smoking.MethodsA national sample of 11–19-year-old males (n = 228) completed an online survey in November 2011. We recruited participants through their parents, who were members of a panel of U.S. households constructed using random-digit dialing and addressed-based sampling.ResultsOnly two participants (< 1%) had previously tried e-cigarettes. Among those who had not tried e-cigarettes, most (67%) had heard of them. Awareness was higher among older and non-Hispanic adolescents. Nearly 1 in 5 (18%) participants were willing to try either a plain or flavored e-cigarette, but willingness to try plain versus flavored varieties did not differ. Smokers were more willing to try any e-cigarette than nonsmokers (74% vs. 13%; OR 10.25, 95% CI 2.88, 36.46). Nonsmokers who had more negative beliefs about the typical smoker were less willing to try e-cigarettes (OR .58, 95% CI .43, .79).ConclusionsMost adolescent males were aware of e-cigarettes, and a substantial minority were willing to try them. Given that even experimentation with e-cigarettes could lead to nicotine dependence and subsequent use of other tobacco products, regulatory and behavioral interventions are needed to prevent “gateway” use by adolescent nonsmokers. Campaigns promoting negative images of smokers or FDA bans on sales to youth may help deter use.     

The SELMA Study: A Birth Cohort Study in Sweden Following More Than 2000 Mother-Child Pairs

Background: This paper describes the background, aim and study design for the Swedish SELMA study that aimed to investigate the importance of early life exposure during pregnancy and infancy to environmental factors with a major focus on endocrine disrupting chemicals for multiple chronic diseases/disorders in offspring. Methods: The cohort was established by recruiting women in the 10th week of pregnancy. Blood and urine from the pregnant women and the child and air and dust from home environment from pregnancy and infancy period have been collected. Questionnaires were used to collect information on life styles, socio-economic status, living conditions, diet and medical history. Results: Of the 8394 reported pregnant women, 6658 were invited to participate in the study. Among the invited women, 2582 (39%) agreed to participate. Of the 4076 (61%) non-participants, 2091 women were invited to a non-respondent questionnaire in order to examine possible selection bias. We found a self-selection bias in the established cohort when compared with the non-participant group, e.g. participating families did smoke less (14% vs. 19%), had more frequent asthma and allergy symptoms in the family (58% vs. 38%), as well as higher education among the mothers (51% vs. 36%) and more often lived in single-family houses (67% vs. 60%). Conclusions: These findings indicate that the participating families do not fully represent the study population and thus, the exposure in this population. However, there is no obvious reason that this selection bias will have an impact on identification of environmental risk factors.     

Intimate partner violence prevalence estimation using telephone surveys: understanding the effect of nonresponse bias

To assess the effect of nonresponse bias in telephone prevalence studies of intimate partner violence, the authors asked women visiting a health center in Albany, New York, during 1998 about their willingness to participate in telephone surveys. Women physically victimized by a male partner were more likely than other women to say they would participate in telephone surveys (66.7% vs. 44.4%, p = 0.03). Among women severely victimized, those living with their partner were less willing to participate than those not cohabiting (45.5% vs. 91.7%, p = 0.03). Including questions about willingness to participate in telephone surveys in studies of other kinds may be a useful method of identifying nonresponse bias.