Controlled Clinical Trials 22:238–247 (2001)© Elsevier Science Inc. 2001 0197-2456/01/$–see front matter655 Avenue of the Americas, New York, NY 10010 PII S0197-2456(01)00133-7

Locating Study Subjects: Predictors and Successful Search Strategies with Inmates Released from a U.S. County Jail

Enrique Menendez, MD, Mary Castle White, MPH, PhD, and Jacqueline P. Tulsky, MD

Department of Community Health Systems, School of Nursing, University of California, San Francisco, California (E.M., M.C.W.) and AIDS Division, Internal Medicine, University of California, San Francisco, California (J.P.T.)

ABSTRACT:

Minimizing loss to follow-up in longitudinal studies is critical. The purpose of thisstudy was to examine the ability to locate subjects recently released from jail, identifypredictors of being able to find a subject, and describe effective search strategies for thisunique population. The sample for this cohort study included study subjects who weresought for interview after release from jail. Inmates in the San Francisco City andCounty Jail were enrolled in a randomized trial of incentives to improve follow-up fortuberculosis therapy after release from jail. Sociodemographic, health-related, and ex-tensive locating information was collected during baseline interviews in jail. The mainoutcome was successful location of the subject. Study personnel recorded data on thenumber and nature of attempts made to find subjects in order to describe successfulsearch strategies. Of 254 persons sought for the postrelease interview, 188 (74.0%) werefound. Primary English speakers were more likely than Spanish speakers to be found(relative risk: 3.2, 95% confidence interval: 1.5–6.7,

p

5

0.002). Nearly one quarter of sub-jects (24%) were found back in jail, and the remainder were found in the community.Phone calls and letters to the subjects, and personal contacts to family and friends weresuccessful strategies for 53% of the subjects. Seeking persons in programs, such as shel-ters and drug and alcohol programs, was successful in finding 18% of English-speakingsubjects. Outreach efforts in sections of the city where Latinos spent time, includingpopular restaurants and community gathering places, were successful in finding 13% ofSpanish-speaking subjects. We conclude that study subjects released from jails can besuccessfully located using well-defined search protocols tailored to the ethnicity of thesample and including a variety of strategies. Employment of bilingual personnel isimportant when a large proportion of subjects is monolingual and non-English speaking.

Control Clin Trials

2001;22:238–247 © Elsevier Science Inc. 2001

KEY WORDS:

Jail, corrections, attrition, longitudinal study methods, locating study subjects, follow-up

Address reprint requests to: Dr. M.C. White, Associate Professor, Department of Community Health Systems, School of Nursing, University of California, San Francisco, 2 Kirkham St., San Francisco, CA 94143-0608 (mcwhite@itsa.ucsf.edu)

Received October 10, 2000; accepted February 20, 2001

Locating Study Subjects

239

INTRODUCTION

Loss to follow-up can be a critical limiting factor in study design, analysis,and interpretation of longitudinal study data, resulting in reduced power andthe potential for biased findings [1–3]. Characteristics of subjects associatedwith high loss to follow-up include alcoholism, substance abuse, and beingfrom a marginalized population, such as homeless persons [4–9]. While somestudies have focused on specific marginalized populations, none have de-scribed incarcerated populations who are released from custody to the generalcommunity. The population of inmates is particularly challenging, becausecharacteristics of the inmates themselves and the nature of incarceration mayinfluence success in follow-up. For example, Hispanics comprised 15.5% of jailinmates in 1999, and were 2.3 times as likely to be held in jail as were whitenon-Hispanics [10]. Inmates in general and Latinos in particular may have ad-ditional characteristics that could impact follow-up, such as language, accul-turation, and immigration status.

To minimize the loss of study subjects, research personnel have developed anumber of strategies to find subjects in the community [1, 3, 11–13]. Thesestrategies can collectively be called the search protocol, defined as the group ofefforts aimed at finding study subjects for follow-up, to minimize attrition of astudy sample, continue therapeutic or diagnostic protocols in clinical studies,and measure outcomes. McKenzie et al., in a review of research to date, synthe-sized key elements important in follow-up of marginalized populations [14].They were: collection of contact information; thorough organization and docu-mentation of efforts to locate subjects; staff training and support; use of phone,mail, and incentives; establishing rapport; assurance of confidentiality; contact-ing agencies to help locate persons; fieldwork; and attention to safety concerns.Characteristics of the study team and the protocol have also been identified asimportant in successful location of subjects for follow-up. Areán and Gal-lagher-Thompson suggested that cultural competence is the most important is-sue in studies of an ethnic minority population, including attention to thesetting and to the language and culture of the staff administering study proto-cols [15].

The need to examine effective strategies to ensure retention of study subjectsis critical, particularly among subjects from the population of incarcerated per-sons. The purpose of this study, as part of a larger clinical trial on adherence totherapy for latent tuberculosis (TB) infection, was to examine the ability to locatestudy subjects recently released from jail, identify predictors of being able tofind subjects, and describe effective search protocols for this unique population.

METHODS

Design, Setting, and Sample

A prospective cohort design was used in this study of inmates in the SanFrancisco City and County Jail. The sample included inmates enrolled in theTuberculosis Prevention Project, a clinical trial of interventions to improvecompletion of therapy for latent TB infection after release. The trial occurredbetween 1997 and 2000 (manuscript in review). The protocol tested educationversus the promise of a financial incentive, on two outcomes: first visit to the

240

E. Menendez et al.

County TB clinic within 1 month of release and completion of therapy for la-tent TB infection. Inclusion criteria were English or Spanish language, agree-ment to be in the study, and treatment for latent TB infection. Excluded wereinmates who were determined by the Sheriff’s personnel to be violent or tohave serious psychiatric illness. Inmates in jail under care for human immuno-deficiency virus (HIV) or AIDS were excluded because they were in specialprograms for follow-up after release.

Study subjects were informed at enrollment that they would be contactedfor a follow-up interview after release. As part of the trial protocol, attemptswere made to interview subjects at the time of the first TB clinic visit. The sam-ple for this analysis includes those who were sought for this follow-up inter-view in the community, including those who came to the TB clinic but did notremain long enough for study personnel to meet with them and those who didnot go to the TB clinic at all.

All study procedures, including consents, the format for gathering contactinformation, and releases to contact agency personnel, were approved by theUniversity of California, San Francisco, Institutional Review (Human Subjects)Board.

Data and Data Collection

Independent Variables

Sociodemographic and lifestyle variables were collected at the baseline in-terview, which was conducted in English or Spanish according to the subject’spreference. Information was obtained by self-report, and questions such asthose regarding a perceived problem with drugs or alcohol were asked in gen-eral terms to protect inmates who were in trial when interviewed. Immigrantstatus and reason for incarceration were not available to us. Homelessness inthe month before jail was determined by a positive response to the question “Inthe month before coming to jail, did you spend any night on the street or in ashelter, in other words, homeless?” Questions were also asked that would helpthe study team find the subjects after release. These questions regarded name,birth date, nickname, and Social Security number. Because inmates sometimesused another name during incarceration, time was spent asking about all othernames used in the community. Other information collected included type ofhousing before jail and anticipated residence after release, along with ad-dresses and phone and pager numbers.

Data collected on contacts who might assist in locating the subjects fell intothe three categories identified by Goldstein et al. [4]. They were: informal, orassociations of high intimacy and immediacy such as family and friends; for-mal, or organizations of low intimacy such as landlords and places of employ-ment; and control, such as corrections and public or private treatmentprograms. For informal contacts, subjects were asked to provide family or part-ner names, addresses, and phone numbers. Family members, in particularmothers, have been identified as persons who reliably maintained contact andgot messages to clients who were lost to follow-up in a study of drug abusers [16].

Among formal networks, subjects were asked about shelters and other tem-porary living residences that might be used after release from jail. Questionswere asked about General Assistance and other public assistance programs

Locating Study Subjects

241

and the locations where subjects reported picking up checks. Employment in-formation included where the person worked and whether study personnelcould attempt to contact the subject, or his or her employer, at the work setting.In addition, research personnel asked about places where subjects frequently“hang out,” such as restaurants, gyms, parks, or community settings wherepersons congregate to find day work.

Among control networks, research assistants asked information about pro-bation or parole officers and medical care settings such as drug and alcoholtreatment programs. And finally, research personnel noted additional physicalcharacteristics of the inmate and any other relevant information that the inter-viewer and the subject thought might be useful when attempting to locate himor her after release.

Information to help in the search was collected for all study subjects in an in-teractive manner such that the subject could understand and see the informa-tion being recorded. Questions such as “If we tried all the contacts you gave us,and still couldn’t find you, if you were me, where would you try next?” wereasked in a way that subjects were able to participate in thinking about theirown search strategy. Subjects understood, as part of the consent process, thatstudy staff would not mention TB or jail in any written or verbal communica-tions with contacts or agencies.

For the purpose of quantifying efforts to locate subjects, a search attemptwas defined as an effort to find the subject based on the information obtainedat enrollment into the clinical trial. These included a phone call; visit to a sitenamed by the subject, such as a restaurant, home, or other community location;and the mailing of letters to subjects or contacts. General activities, such as reg-ular contact with jail personnel to determine if the subject had returned to jail,were not counted when attempts were summed for each individual.

Dependent Variable

The study outcome was whether the subject was located, regardless ofwhether the interview for the clinical trial was completed. The subject was re-corded as found if he or she was contacted in person or by phone, with confir-mation of identifying information to validate identity, as agreed upon duringthe baseline interview.

Search Protocol

Information from each study subject was maintained on a spreadsheet thatserved as the mechanism to determine when subjects were released from jailand eligible for follow-up. Once a subject missed an interview at the TB clinic,study staff initiated the search protocol. The first efforts took place in the officeand included phone calls to subjects or their contacts. If the data collected fromthe subject did not include phone numbers, letters were sent to the last knownaddress, which has been shown to be an effective strategy [17]. If the subjecthad reported homelessness or did not give information indicating stable hous-ing, shelters and other agencies were contacted on a weekly basis, and letterswere provided to agency personnel to forward to the subject. Outreach effortsincluded visits to community sites, as provided in the baseline interview, to

242

E. Menendez et al.

look for the subject directly or to leave a letter with a person previously desig-nated by the subject.

After all attempts to find the subject were exhausted, research assistantscontinued to review jail records weekly in the event the subject was re-arrested. Finally, the subject was considered lost if study personnel were un-able to find him or her after 5 months of efforts (6 months after release fromjail), using all available data and contacts. All search efforts were recorded ondata collection sheets kept for each subject and updated weekly, including thetype of effort, date, time, and result.

Throughout the study 75% of personnel were fluent Spanish speakers, basedon previous studies indicating that approximately 60% of the target populationfor the clinical trial were monolingual Spanish-speaking Latinos [18, 19]. Thestudy team was trained and monitored for consistent use of the search proto-col. Regular meetings with staff to review search efforts and solve problemsserved both to ensure that an adequate effort was made for each person and tomaintain morale among the research personnel. Informal competition amongstudy personnel, first reported by Craig as successful in maintaining enthusi-asm, especially when progress was slow [16], was used as a positive tool dur-ing these meetings. The project director coordinated these meetings to reinforcetraining, monitor safety issues, and provide support to the tracking staff.

Data Analysis

Subjects who were found were compared to those who were not found, bysociodemographic characteristics and factors associated with loss to follow-upsuch as drug or alcohol problems, social support, and housing stability. Thesample was described using standard descriptive statistics. Bivariate compari-sons were made, using

t

tests and chi-square analysis, to identify predictors ofbeing found. Variables significant at alpha

5

0.10 were entered into a logisticregression to identify significant predictors while controlling for covariates in-cluding study group from the clinical trial. Analyses were done using SPSS sta-tistical software.

RESULTS

Sample Characteristics and Predictors of Being Found

The sample included 254 subjects, and of these 188 (74.0%) were found.They are described in Table 1 for all subjects who answered baseline questionsabout sociodemographic characteristics. Because of similar results in subcate-gories of marital status, this variable is presented and was analyzed further asa dichotomous variable, partnered (including married or partnered) and un-partnered (never married, separated, divorced, or widowed). The mean age ofthe sample was 30 years. On average, 5.7 attempts were made for those whowere found (median

5

5, range: 1–22), and 10.2 attempts were made for thosewho were not found (median

5

9, range: 3–23).Over half of the subjects (57.8%) were Latino. Subjects who reported that

they were Asian or Latino had the lowest rates of being located (64.3% and66.0%, respectively) as compared to those reporting other ethnic backgrounds.

Locating Study Subjects

243

Persons who indicated a preference for English, versus Spanish, at baseline in-terview were 1.3 times as likely to be found (95% confidence interval: 1.2–1.5,

p

,

0.001). Place of birth (United States versus outside the United States) hadnearly identical results as language preference and was highly correlated withthis variable. Among subjects born outside the United States, longer time in theUnited States was associated with being found for interview. Time in theUnited States was categorized into three levels: 0–5 years,

.

5 years but for-eign-born, and U.S.-born subjects. This ordinal variable was significantly asso-ciated with being found, by the chi-square test for trend (

p

5

0.011) (Table 1). A

Table 1

Characteristics of 254 Study Subjects Sought for Follow-up Interview after Release from Jail, by Whether or Not They Were Found

Characteristic

n

Found (

n

) Found (%)

p

-value

GenderMale 228 167 73.2 nsFemale 26 21 80.8

EthnicityLatino 147 97 66.0 0.001

a

Black 54 47 87.0Other, mixed 26 23 88.5Asian 14 9 64.3White 11 10 90.9Native American 2 2 100.0

Birth countryUnited States 82 70 85.4 0.001

a

Mexico 66 38 57.6Honduras 37 24 64.9El Salvador 18 16 88.9Other Latin America 21 16 76.2Asia/Pacific Islands 17 12 70.6Other 10 9 90.0

Preferred languageEnglish 109 94 86.2

,

0.001Spanish 145 94 64.8

Time in United States0–5 years since immigration 76 50 65.8 0.011

b

.

5 years since immigration 84 60 71.4Born in United States 84 72 85.7

Partnership statusNever married, separated, divorced,

or widowed 167 128 76.7 nsMarried/partnered 87 60 69.0

Housing before jailEver homeless in last month 76 57 75.0 nsNever homeless in last month 175 128 73.1

Employed before jailEmployed in last 3 months 144 109 75.7 nsUnemployed in last 3 months 110 79 71.8

Drugs or alcoholPerceived problem 136 99 72.8 nsNo perceived problem 118 89 75.4

Total 254 188 74.0

a

Chi-square test of the value in this row as compared to all other values.

b

Chi-square test for trend.

244

E. Menendez et al.

stepwise logistic regression analysis indicated that language (preferring En-glish versus Spanish for baseline interview in jail) was the only variable thatpredicted being found (relative risk: 3.2, 95% confidence interval: 1.5–6.7,

p

5

0.002), while controlling for covariates and the clinical trial study group.

Time to Locate Subjects and Effective Strategies

In the subset of 188 persons who were located, the average time to locationwas 61.5 days (median

5

36 days, range: 1–180). Of the 21 women found, on av-erage the time to location was 45 days, as compared to 64 days for the 167 menwho were located, although this difference was not statistically significant. Thissample experienced a high rate of recidivism: overall, nearly one quarter of thesubjects (24%) were found back in jail. Phone and personal contacts (family andfriends) were the most successful strategies for 53% of the sample.

Effective strategies differed among subgroups. Because we found that pre-ferred language, ethnicity, and time in the United States were associated withbeing found, we described the strategies that were successful, stratified bythese variables (Table 2). Data on successful strategies were missing for onesubject. Seeking persons in shelters and drug and alcohol programs were suc-cessful strategies among non-Latinos and English-speaking subjects. Outreachefforts were successful, in particular among Latinos, Spanish-speaking sub-jects, and new immigrants. These efforts focused on sections of the city whereethnic groups spent time, including popular restaurants and community gath-ering places.

DISCUSSION

In a population of study subjects released from jail, we located 74.0% basedon information gathered at baseline. Harlow reported successful tracking to

Table 2

Percent of Persons Found Using Different Search Strategies, by Characteristics Identified as Predictors of Being Found

Predictor Jail Phone/Letter Contacts Programs Outreach

LanguageEnglish (

n

5

93) 26 32 15 18 9Spanish (

n

5

94) 22 34 23 7 13Time in United States

0–5 years (

n

5

50) 24 34 20 8 14

.

5 years (

n

5

60) 17 43 18 8 13Born in United States (

n

5

71) 30 24 20 20 7Ethnicity

Latino (

n

5

97) 24 34 21 9 12Black (

n

5

47) 30 30 15 18 6Other, mixed (

n

5

22) 18 32 23 27 9White (

n

5

10) 30 20 30 20 0Asian, Native American (

n

5

11) 9 54 9 0 27Total (n

5

187) 24 33 20 12 11

Contacts included individuals named by study subject. Programs included shelters, drug and alcoholprograms, TB clinic, and other programs that subjects provided permission to contact. Outreach includedvisits to community settings, including popular restaurants and community gathering places.

Locating Study Subjects

245

completion of TB preventive therapy in 63% [20], and our rates of locating sub-jects were similar to those found by Ludwig et al. [21]. Results from this studyfocus on locating a specific group that has several differences from those stud-ied by others. Released inmates are a group with special characteristics thatmay affect outcome. First, because this sample came from a study of TB in awest coast city, it included a disproportionate number of new immigrants, inparticular from Latin America, and language was an important predictor ofsuccess in locating subjects after release. In addition to cultural differences,they may have had immigration issues that affected their willingness to befound for follow-up.

Predictors of locating subjects were not in complete agreement with otherresearch. Poor social functioning, including unpartnered or unmarried status,predicted loss to follow-up in a sample of 85 alcoholics [8]. Although not statis-tically significant, our findings indicated that persons who were married orpartnered at baseline interview were less likely to be located after release fromjail. In several instances, the subject named a partner as the main or only con-tact, and when approached, this contact person was no longer partnered withthe former inmate. The fact of being in jail may have prompted the dissolutionof the partnership, or, in the extreme, the reason for the person being in jailmay have been related to the relationship. Senturia et al. found that the inabil-ity to provide several alternative contacts was predictive of retention problems[22]. This finding, combined with our results, leads us to believe that probingfor as many contacts as possible is an important strategy, especially if the in-mate provides only one partner as a contact.

Our work also differed from that of MacKenzie et al. in the area of homeless-ness. In their study of alcoholics, residential instability in the follow-up periodpredicted loss to follow-up [8]. In our study, persons who reported homeless-ness before jail were more likely to be found, although the finding did notreach statistical significance. In this case as well, the fact of the jail stay mayhave influenced the ease with which persons were found after release. Personswith a history of homelessness were able to name shelters where they occasion-ally stayed, and these were stable locations as compared to persons who hadone or two contact individuals and private residences that may have becomedisrupted as a result of their jail stay.

Among effective strategies, jail itself has been identified as an important set-ting for locating subjects, in our study and in studies of persons with a historyof time in jail. Whelan et al. found that among high-risk drug users in Oregon,19% were found while they were incarcerated [23], which is similar to our rateof 24% who were found in jail. Ludwig et al. found that the most productivestrategies in the community were mail and telephone contacts with relativesand friends [21]. In our sample of released inmates, known to have high ratesof substance abuse and mental health problems, programs for drug and alco-hol problems were successful places to search for subjects. And finally, in thispredominantly Latino sample, our successful outreach efforts included ques-tions about sections of the city where particular subgroups congregated for so-cial or employment reasons. Sending staff to these areas was a successfulstrategy in particular for those who were new immigrants from Latin Ameri-can countries.

Staff preparation and cultural competence have been cited repeatedly in the

246

E. Menendez et al.

literature and were important in this study. Marmor et al. examined follow-upin the Framingham Children’s Study and found that attitudes of staff, feedbackto subjects, and the staff’s handling of questions and problems were importantpredictors of continued participation [1]. They concluded that quality of thecommunication with study subjects and the subjects’ perceived importance ofthe research were key factors in minimizing loss to follow-up. In a study ofdrug abusers in St. Louis, Cottler et al. identified persistence, creative team-work, and a comprehensive searching strategy as critical factors in successfulfollow-up [24]. Krieger et al. found that in a clinical trial in which enhancedsearching and follow-up were parts of the intervention, community healthworkers of the same ethnicity and from the same communities may have ac-counted for the effectiveness of outreach [25]. Desland and Batey located 76%of heroin users for follow-up interviews, citing collection of good and reliableinformation, ethical practices in informing subjects of searching methods, con-fidentiality of the methods, and using creativity and flexibility in searching forsubjects as reasons for their success [3]. In our study, research personnel werebilingual and bicultural, search protocols were consistent and monitored, andthe study team was trained, monitored, and encouraged and maintained en-thusiasm throughout the study. We believe these are critically important as-pects of a study protocol, both to obtain sufficient contact information and toapproach contacts appropriately and successfully in the community.

Locating study subjects in longitudinal research is critical to achieve suffi-cient sample size for analysis and to make conclusions that can be consideredvalid. The population of released inmates shares characteristics, such as unsta-ble housing, drug and alcohol problems, and unpartnered status, that havebeen identified as predictors of loss to follow-up. However, in this study thesecharacteristics did not predict loss to follow-up, as incarceration per se seemedto influence the ability to locate study subjects. For this population, therefore,attention should be paid to collection of adequate residential and social contactinformation. Specific ethnic groups, such as Latinos in this study, differ in thesocial contacts and strategies that will be most successful. For those who prefera language other than English, special efforts include employment of biculturalas well as bilingual personnel. Identification of those who are likely to be diffi-cult to find can help project managers allocate resources appropriately.

This project was funded by the National Institutes of Health, NINR (R01 NR04456).

REFERENCES

1. Marmor J, Oliveira S, Donahue R, et al. Factors encouraging cohort maintenance in alongitudinal study.

J Clin Epidemiol

1991;44:531–535.2. Freedman D, Thornton A, Camburn D. Maintaining response rates in longitudinal

studies.

Soc Methods Res

1980;9:87–98.3. Desland M, Batey R. High retention rates within a prospective study of heroin us-

ers.

Br J Addict

1991;86:859–865.4. Goldstein PJ, Abbott W, Paige W, Sobel I, Soto F. Tracking procedures in follow-up

studies of drug abusers.

Am J Drug Alcohol Abuse

1977;4:21–30.5. Bale R, Cabrera S, Brown J. Follow-up evaluation of drug abuse treatment.

Am JDrug Alcohol Abuse

1977;4:233–249.

Locating Study Subjects

247

6. Moos R, Bliss F. Difficulty of follow-up and outcome of alcoholism treatment.

J StudAlcohol

1978;39:473–490.7. Klonoff-Cohen H. Tracking strategies involving fourteen sources for locating a tran-

sient study sample: Parents of sudden infant death syndrome infants and control in-fants.

Am J Epidemiol

1996;144:98–101.8. MacKenzie A, Funderburk FR, Allen RP, Stefan R. The characteristics of alcoholics

frequently lost to follow up.

J Stud Alcohol

1987;48:119–123.9. Pollio DE, Thompson SJ, North CS. Agency-based tracking of difficult-to-follow

populations: Runaway and homeless youth programs in St. Louis, Missouri.

Com-munity Ment Health J

2000;36:247–258.10. Beck AJ. Prison and jail inmates at midyear 1999. In:

Bureau of Justice Statistics Bulle-tin.

Washington, D.C.: U.S. Dept of Justice; April, 2000. NIJ 181643.11. Wright J, Lampton A, Devine JA. Tracking non-traditional populations in longitudi-

nal studies.

Evaluation and Program Planning

1995;18:267–277.12. Given B, Keilman L, Collins C, Given W. Strategies to minimize attrition in longitu-

dinal studies.

Nurs Res

1990;39:184–186.13. Hough R, Tarke H, Renker V, Shields P, Glatstein J. Recruitment and retention of

homeless mentally ill participants in research.

J Consult Clin Psychol

1996;64:881–891.14. McKenzie M, Peterson Tulsky J, Long HL, Chesney M, Moss A. Tracking and fol-

low-up of marginalized populations: A review.

J Health Care Poor Underserved

1999;10:409–429.15. Areán PA, Gallagher-Thompson D. Issues and recommendations for the recruit-

ment and retention of older ethnic minority adults into clinical research.

J ConsultClin Psychol

1996;64:875–880.16. Craig R. Locating drug addicts who have dropped out of treatment.

Hosp Commu-nity Psychiatry

1979;30:402–404.17. Eckland BK. Retrieving mobile cases in longitudinal surveys.

Public Opinion Quart

1968;32:51–64.18. Tulsky JP, White MC, Dawson C, et al. Screening for tuberculosis in jail and clinic

follow-up after release.

Am J Public Health

1998;88:223–226.19. White MC, Tulsky JP, Reilly P, et al. A clinical trial of a financial incentive to go to

the tuberculosis clinic for isoniazid after release from jail.

Int J Tuberc Lung Dis

1998;2:506–512.20. Harlow T. TB net tracking network provides continuity of care for mobile TB pa-

tients.

Am J Public Health

1999;89:1581–1582.21. Ludwig AM, Levine J, Stark LH.

LSD and Alcoholism: A Clinical Study of Treatment Ef-ficacy.

Springfield, Illinois: Thomas, Inc.; 1970.22. Senturia Y, Mortimer K, Baker D, Joseph C, Wedner J. Successful techniques for re-

tention of study participants in an inner city population.

Control Clin Trials

1998;19:544–554.23. Whelan V, McBride D, Colby R. Public health department tracking of high-risk drug

users.

Public Health Rep

1993;108:643–645.24. Cottler L, Compton W, Ben-Abdallah A, Horne M, Claverie D. Achieving 96.6 per-

cent follow-up rate in a longitudinal study of drug abusers.

Drug Alcohol Depend

1996;41:209–217.25. Krieger J, Collier C, Song L, Martin D. Linking community based blood pressure

measurement to clinical care: A randomized controlled trial of outreach and track-ing by community health workers.

Am J Public Health

1999;89:856–861.