Aftennonb multtpleforms of error canimprovetheeffectiveness of suweysin evaluntion. Sourcesof SurveyError: Implicationsfor EvaluationStudies MarcT. Brayerman Surveys constitute one of the most important data collection tools available in evaluation. For example, sample surveys are used ro gauge public opinion in policy research and are used with specialized target populations in needs assessmentstudies. Self-administered survey questionnaires are used in classroom settings to assessdifferences between program participanrs and control groups. Structured survey interviews are used to assessthe effectiveness of clinical treatments. Surveys are used in longitudinal studies to track long,term change in targeted populations. Although these examples differ in many ways, they all share a focus on direct questioning, in a variety of manifestations, to leam about people-their behaviors, their attitudes, and other aspecrsof their lives. Much of the focus in this volume is on sample surveys, which use systematic procedures for selecting probability samples from populations. Program evaluators probably use sample surveys a good deal more now than in years past, due to recent developments that often identify entire communities as program targets. These developments include greater attention to media campaigns as a form of social program (for example, Flay, Kessler, and Utts, i991) and the current trend toward comprehensive communitywide interventions ([or example,Jackson,Altman, Howard-Pitney,and Farquhar, l9B9). Randomized population surveys have been used in the evaluations of several I thank Robert Groves for his valuable and insightful comments on an earlier draft of this chapter. Nff DrREOroNs FoR EvALUAfloN, no, 70, Summer 1996 @Jossey,Brc publishere 77 1B ADVAN(-I:s rNSutrvlr l,ll.:.l.Atr( ti SouRcrsoF SURVEv ERRoR nationallypKtlnillctttc(rrrllllrlrttly llr:illtlrltrl(]tvcrrtr()lt such as the Minl)l'oiects, n e so taHear t 1{ c ' alr h[ ) 1 ' o 1 1 r;rrn tl ,i rr:p k ri rrrri lo rl rc rs ,1994) and the N ati onal (.()fulMll rlialr ((.(llvl Mll' l{csearchGroup, rgg5). NeverCancer InstirLrr(:'s theless,as Hi,:rrrypoiritr (rrr iri (.lraptcr()ne, the potential of population surveys in evalirarionrt's(,';rlt'lr lerrrrir.rslargelyuntapped, and he suggeststhat surveys can lte ttseclto lrrovrrlee()ntextfor evaluation recommendationsand to communicluc irrlirr"rnat i()n to the public. Many lrl,gr lrrr evaluarions,particularly those that involve experimental or quasi-cxpcrirtrcrrraldcsigns,do not use population suweys, and their selection critclia lor irrcludingindividuals as participantsdo not incorporatethe technicul issucsrelated to sampling from a larger population. However, all evaluations that use questionnaires in any form can benefit from the current research regarding measurement errors, that is, errors of observation that involvc the interviewer, respondent, instrument, or mode. My aim in this chap(er is to provide a briel overviewof some recentadvancesin survey ,.raur.'h, within the contexr of different kinds of survey error and with partiiular attention to the applicability of the findings for program evaluation. I have drawn on my own areaof evaluationwork for examples;thus, most of them concern health promotion programs and educational programs Componentsof SurveyError Groves (1989) presentsa schematicmodel of survey error attributable to survey design features, which builds on formulations of total suwey error by Kish (1965) and Andersen, Kasper,Frankel, and Associates (1979).In thesemodels, the mean square error-the total error associatedwith a survey statisticconsists of two major families of subcomponents: error due to statistical variance and error due to bias. vaiance error arises from the unavoidable variability in the target statistic acrosssamples of the populatlon units or, more generally, from any aspect of the survey design that will vary over replications. Thus, variability will be found acrossindividuals within rhe populatibn (which produces sampling error), acrossresponsesfrom a single indMdual to the same question or set of questions (which produces levels of resr-retesrreliability), across test items sampled from a content domain (which produces levels of parallel forms reliability), and so on. These error components have an expected vaiue of zerc, that is, they are unbiased with respect ro the popuiarion parameter being estimated. By contrast, bias refers to fixed erto. to-ponent; that are specific to a particular survey design and leads to sample sratisticsthat can be expected to differ from their respective population pu.u-"t"., by some nonzero, though typically unknown, amount. Researchersseek to minimize variance error and to eliminate blas error within the consiraints of acceprableresearchcosts(seeGroves,lg8g). in addition, researcherstypically seek also to assessthe extent of both kinds of error in their data and to calculate what effectsthese may have on the statistical estimates. Postsurveyprocedurescan then be applied to compensatefor the estimated levels of error. 19 within the respective sets of error due to variance and error due to bias, Groves (1989) distinguishes further between errors oJnonobsewationand errors oJobsewationt rhe first categoryincludes errors due to coverage,nonresponse, and sampling. The second category includes errors due to interviewers, respondents,instruments, and modes. This model frames the discussion that follows. Errors of Nonobservation Broadly speaking, errors of nonobservation are due to the fact that measurements on some eligible persons have not been made. This occurs when some persons are not identified prior to sampling (coverage error), when some persons refuse to be interviewed or cannot be reached,(nonresponseerror), and vThsn-25 is inherent in the concept of sampling-some persons are excluded from the selected sample (sampling error). Coverage Errors. Coverage errors arise because units of the target population that should be eligible for the survey have nor been included in the sampling frame. To illustrate: one of the most widely appreciaredexamplesof coverageerror is the fact that general population surveys conducted by telephone exclude householdsrhat do not have telephoneservice.This is a particular concern in sun'eys of populations known to have relatively high proportions of nontelephone househoids, including low-income populations and rural populations, among others. Since this exclusion is a fixed component of any telephone survey design, if nontelephone households differ from telephone households on the survey variables being measured (as they frequently do), the resulting error will constiture a bias in the design rather than a component of statistical variance. Poststratification adjustments on demographic variablessuch as age,sex, and educational attainment are often made to compensatefor biasesin coverage. When lists or registers,r. ,rr.d as sampling frames, a different probiem arises-ineligible target units may be included. In a general population mail suwey that was part of a community-based health program evaluation in Austraiia and used elecroral registers as the frame, Day, Dunt, and Day (i995) found that relephone and personal visit contacts with nonrespondents increasedthe proportion of units deemed ineligible from B percent to 12 percent of the registerentries.Attempting to obtain an accurateestimateof ineligibles is important for obtaining accuratecalculationsof responserates,which otherwise would be artificially low. Nonresponse Errors. Nonresponse errors occur when individuals selectedfrom the frame are not ultimately representedin the data set because they refuse to participate,cannot be reached,or are unable to respond. Survey researchershave expressedconcern that nonresponse is becoming a grearer problem for population surveys than it has been in the past (for example, Bradburn, 1992). Becauseof the obviously high porenrial for bias rhat resuhs when alarge proportion of the selected sample is noncooperative, a greatdeal of researchhas been conducted on techniques for minimizing nonresoonseand . lt l j l l r * i i i ; ' : r , : ; t 1 . !" " r iEi,ifi lia :r 1 ir i ,t iillltt3tiiip, litr nrt:;rrirrittll;rttrl Ir;t it i..iiripi:l';rlirl(itt)r,t:s(seeChapterFive) present $rvcfal rhr:rrrcrir';rl it!r'.r tlr:t ;trlrlresswhy contactedindividuals lrr.i"il.!.rq and they examine those permight cithcr gfrnt ol rr-h1.,,. :t =rrfr.rryr.rq11q51, spectivesin liglrr trl tl;rtirlrlrfri tlrr=I f .5. t.t:nsusand severalnational surveys. Ilcsprirrst'Itrrlrl lirrvrr. lr.:lt;!r'r'llcrshave long been interested in refiner'csponse rates.This interestmay be greatest ments (rf lilir r"rluli':,!i I rr',rrrru;'-(' irt tltc cit:,r'ril rrr;rilt't I illr:it r{rtutaircs,since difficulty in obtaining high response i.r1tt:r't;il llrr: illlrrlc:,.tnr.'irlruess of this survey mode as compared to telephone (!r lt{'ll,.ift;tlilrlf l \ii(:w tnodes.The most comprehensiveset of recommendali*!l:; ltri lr;r,rinli;ing mail responseratesis probably Dillmans (1978) Total Ili.l1gt' Miithrtd. ln addition, numerous researchreviews on factors affecting Irliill :.r.nv(:y responserateshave appearedin recent years (for example, Church, l!)tlri; l;ox, Crask, and Kim, I9BB; Yammarino, Skinner, and Childers, l99l). facttrrs consistently found to predict higher responserates include repeated contacts (preliminary notification and follow-ups), monetary incentives, inclusion of a postage-paid return envelope, certain tlpes of cover letter appeals, and a questionnaire length of four pages or less (Yammarino, Skinner, and Childers, 1991). For survey projecrs that use household interview and telephone interview modes, detailed practical adnce on maximizing responserates can be found in Morton-Williams (1993). Adlustingfor NonresponseBias. Apart from maximizing respondent cooperatlon, researchersneed methods to estimate the degree of nonresponse bias and make adjustmentsin their sample statistics.Groves (1989) provides technical treatments of statistical adjustment approaches, including weighting procedures to adjust for nonresponding individuals and households, and imputation procedures to estimate values of misslng items. One perennial problem for survey researchersis determining the source of information that will serve as the basis for nonresponse estimates.Lin and Schaeffer(1995) analyze two competing hypotheses about survey nonparticipants that are often used to guide estimation measures.The first assumesthat nonparticipants are similar to late respondersand, in fact, fall at the end of a "continuum of resistance"; thus, estimates of nonresponse bias are derived from the data collected from late responders. The second hypothesis assumesthat there are distinct categoriesof nonparticipants, such as refusersand hard-to-contact individuals; estimates for refusers are derived from the number of temporary refusers (those who finally did participate), and estimatesfor the hard to contact are derived from participants who were reached only after several callback attempts. However, Lin and Schaefferconclude that neither model is entirely sufficient to pror,rde a suitable estimate of bias. Accessto Respondents.ln addition to the behanoral trends regarding survey refusals, new obstaclesto gaining accessto respondents have resulted from a number of technological developments. The most significant of these is probably the telephone answering machine. Some researchershave concluded that answering machines do not, as yet, constitute a serious problem for survey research(Oldendick and Link, 1994; Xu, Bates,and Schweitzer,1993). But SouRcesor SunvryEnnon. 2I the possibility of serious bias is introduced if answering machines restrict access to some segments of the population in comparison with others. This was, in fact, found by Oldendick and Link, who reported that households using answering machines to screen unwanted calls tend to be characteized by higher family income and higher levels of educarion. However, Xu, Bates, and Schweitzer found that households with answering machines were more llkely to complete an interview. They also found that leaving messages resulted in higher ultimate participation rates for househoids with machines, perhaps by performing a function similar to the use of advance letters. Most researchersstudying this topic believe that answering machines may well become a greaterthreat to survey research in the future, due to sharp increases in recent years in the number of households owning machines and using them to screencalls. Sampling Errors. Sampling errors occur because the elements of the frame population'(that is, the respondents) differ on the variable of interest, and different samples selected according to the sampling design will include different combinations of those population elements. lf other kinds of error (coverage,nonresponse, and observational errors) do not exist, the difference between any given sample statistic and the statistic'strue population value will constitute sampling error. As Grbves (1989, chap. 6) states,mosr staristical work on sampling error concerns errors of vanance. Certain sampling designs (for example, those involvlng stratificarion) are more efficient than others (for example, simple random sampling) in reducing error. However, the danger of sampling bias also exists when selection procedures are incorrectly applied in systematic ways or when some individuals in the sampling frame have a zero chance of selection, as occurs in nonprobability or quota samples. Henry (1990) provides a practlcal overview of sampling approachesihat can'be used in research planning. Sampling error is probably the most well studied of ail of the different forms of error, and a good deal of effort is expended in survey deslgns to control it. It has received this attention because it is susceptibie to control through staiistical estimation procedures, and design aiternatives can be developed that yield projections of both precision and cost (Groves, 1989). For example, as Groves notes, almost all major surveys use sampling designs that include complex procedures to reduce either sampling error or research costs (procedures such as stratification, clustering, and unequal probabilities of selection).The other forms of survey error are not so easily predicted, measured, or controlled, but in recent years, researchershave begun to energetically addressthese other design problems that are more resistantto solution. Errors of Observation Errors o[ obsewation-also referred to by Groves (1989, p.295) as "measurement errors"-are due to the fact that recorded measurementsdo not reflect the true values of the variablesthey represent. Such discrepanciesare caused A r ' \ , \ r r r l : \ l N S t t R V[:Yllt.:' t ir ltt tt lly factors residing in thc it'tlr:rvirurr:t,llte t'(:slxrndent,the wording or organization of the instrumc:nt,atld l[rr, tnudc ol survey administration. errors are due to the effectsof individInterviewer lirrors. lrr[r:r'\,rt'wr:r ;rriellaee-to-facesurveys.Srokesand Yeh (1988) ual interviewr:r'sirr l-t:ltl.rlrnttc wlty sui:h interviewer effectsmay occur. First, interviewspecify forrr rr;ti;rir-rs ers n1:lylrl r,;rrelc..Fa rrr,negligcntin following directions and protocols. Second, or personal mannerismsthat affectrespontlrry rrr.rvhirri{ r,r'*rritl L:hnrircteristics '1'lrird,their demographiccharacteristics,such as race,age,and rh'r11.:;' ;!r!:,!vr,'rr, 1r'x, njtilyrrllci:l the answersof some respondents.Indeed, a good deal of inveslrJrr Ir.r:;locused on the potential biasesarising from fixed interviewer charnl.,r.rl tr!:lr:ri:jtic:s such as gender (Kane and Macaulay 1993) and vocal characteristics (( )kscnberg and Cannell, 19BB). Fourth, interviewers may vary in rheir prorluction of nonresponse, in terms of both overall nonresponse levels and tlpes of persons who refuse their requests to panicipate. The first three reasons are tlpes of measurement errors; the fourth is an indirect influence of interviewers on total error through their contributions to patterns of nonobservation. The issue of interviewer-related error has drawn considerable attention from survey researchers,but apart from pointing out the need for thorough interviewer preparation, the evaluation literature has virtually ignored the topic. (One welcome exception is provided by Catania and others, 1990, who, within the context of AIDS prevention programs, discuss the evidence for faceio-face interyiewer gender effects on reports o[ sexual behavior.) There are probably several reasons for this lack of attention, including the facts that few program evaluation projects use a large number of interviewers for data collection and that the size of interviewer effectsin a data set is very difficult to estimate if interviewers have not been treated as a randomly assignedindependent variable. Respondent Errors. Respondenteffors are due to processesor characteristics inherent in the respondent. In unbiased form, they consist of inconsistency or unreliability in responding. Biases in respondent error can be introduced by such factors as deliberate misreporting, the activation of response sets (such as social desirability; see Chapter Four by Dillman and his colleagues),motivational states,and characteristickinds of memory retrieval errors for a particular task. Much of the researchon respondent error is informed by cognitive psychological models of the processesthat come into play when people answer questions (for example, Sudman, Bradburn, and Schwarz,1996). The attempt to link survey design with findings from cognitive psychology has been one of the most robust and important directions in survey researchsince the early l980s. (SeeJobe and Mingay, 1991, for a historical perspective.)Only a few examples can be cited here. One line of investigation concerns human memory for autobiographical information (Schwarz and Sudman,1994). For example, Blair and Ganesh (1991) investigated the differencesamong three different ways to ask about pasl events: absolutefrequencymeasures("During the past year, how many times Souncrs oF SURVEyERRoR 23 did you charge a purchase to your accounr?"), rate oJfrequencymeasures("During the past year, about how often did you charge purchases to your account?"), and inter.val-basedJrequency ('When was the last rime you measures charged a purchase to your account?").Although these variations on a single question might be expected to yield highly comparable results, resuhs showed that respondents provide substantially higher frequency esrimares when answering interval-based measures. One reason for the varied results may be that the different formars are differentially susceptible to respondents' misjudging the time of occurrence of an evenr, a kind of error called telescoping. Pearson, Ross, and Dawes (1992) cite studies indicating thar peoples recall of their own personal characteristics at specified times in the past is distorted to conform with their implicit theories about consistency or change in the domain of interest. Thus, when their current responsesare compared with their previous responses on a topic, respondents often do not accurately remember their previous status with regard to attitudes (for example, evaluations of dating partners), behaviors (substance use), and pain (intensiry rarings of chronic headaches). Pearson, Ross, and Dawes argue that these differences are based on memory distortions rather than attempts to mislead. Respondents inferentially construct information about their past using their implicit theories, which are often inaccurate,as blues. ln a final example of cognitively based lines of investigation, Krosnick (1991) has examined the factors rhar mighr derermine rhe amounr of cognitive effort respondents wlll expend in answering survey questions. In Chapter Three, he and his colleagues presenr findings from a series of empirical investigations that seek to identify possible determinants of respondents'Ievelsof cognitive effort. The topic of human cognition in relation to survey methods really stands at the interseciion between respondent error and instrument error (discussed in the next section) because it relates the question design task to characteristic patterns and limitations of human thought. As the cited researchexamples show, these errors stem from particular forrns of response to particular manipulationsoI question presenration. Instrument Errors. Instrument errors are related to uncertaintles in the comprehension and the attributed meaning of questions and are implicated in levels of instrumenr reliabiiity and validity ln its unbiasedform, instrument error will arise because any single scale or instrument can include only a sample of items from the universe of theoretically possible items tapping a skill, attitude, body of knowledge, or other domain. Instrument biasesare due to vagariesof question wording, question structure, and question sequence.In mail surveys, instrument error can also be related to options for the design and format of the questionnaire. Severalexcellent texts exist to provide practical guidance on question wording and instrument construction (for example, Dillman, I97B; Sudman and Bradburn. l9B2). Someiimes, adjustments can be made in the data analysis phase of the researchby removing troublesome items from scalesin post hoc procedures. l.l .{trv*H i | '., l i r 1 l Jl;!,!' l! li- t,l,!Alti 1! how the l{aschmodel can be used lrr (.lrirl'1rcr.:ris, lr.rt{..ranrl;la,!.rn:r1 elr:i!,ur:;r's i!ctlt" lr!i;ti.1ly;;i11,4 trr lelcrtltlyl.la'0i l'litttr"Illsol obtained response,leading to lt lit t t ' t t t r ' t t l o l ' . 1 r tr r 1 .i ,tlr ' r W{)f(JMrlnlrri:i ( !rr)fir5i lq8q) reviewsresearchindicating that a responclcnt will ilu:.iw.il:t iltlf iititll'l(:vcn when he or she perceivesit as ambiguous or tcrms. The researcheris often unaware of these is rrnl;rrniliattvitlt t:r:sctil.tal irrtt:rlltr;!tivli l!t{":,ullll)tionson the part of the lespondent. One must, of ,',r,.,,'n.,5lr'!vl lri tirirtirnizethesepossibilitiesfor respondentmisunderstanding irr rlr,'tlt:li;r-rIitrtr-lpilotingphases of the research.Numerous examplesof sysIr:lrrirli(i ir:rquiryinto question wording are provided by Schuman and Presser t l ()t,l| ) . ulllongm any o th e r s o u rc e s . (.ttnLext Elfects. Context effects (Schwarz and Sudman , 1992) refer to t"csponseerrors createdby variations in the older in which lesponse options are presented on individual closed-ended items or by variations in the order in which the survey questions are presented. Dillman and his colleagues (Chapter Four) provide a fuller description of several forms of response order effects and question order effects wrthin their discussion of survey mode differences.ln addition, Krosnick and his colleagues(Chapter Three) describe several forms of response order effects and how they might be related to respondents' cognitive effort. Muttrple Languages.The meaning that respondents will ascribe to words and phrasing in surveys is clearly a complex and subtle area that has sparked prodigious research.One can imagine, then, that when two oI more languages are involved, the additional linguistic and cross-cultural complexities create enormous potential for new forms of meaning-related error to accrue. McKay and her .oileug.l"r (Chapter Seven) describe the experiences of several largescalesurvey projects in producing and implementing survey iranslations. However, survey translation is an area that has, as yet, seen very little systematic research(seeMarin and Marin, 199i, for a discusslonof some key issuesand concepts). Given the growing linguistic diversity of industrialized societies all over the world, survey translatlon will undoubtedly become a more important topic of research in years to come. Sensitiveltems. Sensitive items, that is, items on which respondents are likely to feel a degree of personal threat (see Lee, 1993), pose a particrrlar problem in that the likelihood of bias is high. Recognition of this possibility is particularly important in the evaluation context because many proglams, especially those for adolescents,deal with sensitive topics such as drug use, sexuality,or risk behanor. Data quality on sensitivequestionsis dependenton a number of design fearures, including question wording, timing of data collection, choice of mode, interviewer decisions, and assulancesof confidentiality Singer, von Thurn, and Miller (1995), in a meta-analysisof studies that involved confidentiality assurances,found that such assurancesimprove response only when the survey items are sensitive, in which case they significantly increase both response rates and data quality. Mail surveys on sensitive SouncrsoF SURVEy ERRoR 25 topics sometimes have the option of guaranteeing complete anonymity to respondents through the removal of all respondent identification information (a condition that is obviously more difficult to promise in the telephone mode and essentially impossible in the face-to-face mode), but guarantees of anonyrnity conflict with the researchers'need to identify nonrespondents for the purpose of targeting follow-up contacts. Biggar and Melbye (1992), in an anonymous sample survey of sexual activity and AIDS attitudes among residents of Copenhagen,Denmark, found that inclusion of a separare,postagepaid "survey completed" card (see Dillman, 1978) did nor lower the initial response rate nor did it appear to influence reports of sexual behavior or atti, tudes. By making follow-up mailings possible, this procedure enabled the survey project to increase its response rate from 45.8 percent after one mailing to 72.8 percent after three mailings. A particular concern regarding respondent bias on sensitive items occurs in evaluations using experimental designs. In addition to recognizing the problems that bias might causefor estimation of outcomes, evaluatorsshould seek to avoid differential bias across treatment and control conditions because this bias can seriously affect interpreiation of program effects.For example, students who participate in a drug prevention education program may become sensitized to repofting drug use. If'they have enloyed the program or feel close to the presenters, they might be inclined to underreport recent drug use at posttest due to a desire to please program staff or to personal embarrassment at hanng violated newly established classroom norms. If students in the control group feel no such compunction, then differential bias will occur, and the program will appear more effectiverhan it is. Alternatively, all groups of students might underreport drug use at pretest due to fears about breach of confidentiality After going through the program, the experimental group students may feel more trusting and therefore repon accurate drug use levels while conrrol group students maintain their previous level of underreporting. This second scenario would createbias in the other direction and make the program appear artificially ineffective. Clearly, extra care in developing questionnaires on sensitive topics is strongly warranted. Mode Errors. Mode errors are created by differencesin the circumstances under which the data are collected: personal interview, telephone interview, or self-administered questionnaire. Dillman and his colleagues(Chapter Four) analyze frequently reported telephone versus mail mode effects and describe how these might stem from fundamental differences between the two formats. They also describe the extent of the researchevidence that supports the existence of the various effects. As Dillman and his colleaguesstate, one reason for the growing research interest in this topic is that many designs mix formats within the same respondent sample. An illustrative example from the evaluation iiterature is the study cited earlier byDay, Dunt, and Day (1995), which used telephone conracts and household visits as two distinct levels of follow-up ro a general population mail survey.Another situation that occurs frequently in evaluation studies entails .l.ft i l l v.{f-!( SoURCESoF SURVEYERRoR t::i l t-l 1t !t- :.\:t:tl{ !ir :,t,A!t!ll eliIler-cntconditions (for example, rLifll:rrnt [!.illuLliir:irlrrtnr.lr:r' strt.vr.yiLi6 l{t youth at schooland mailingsto their parr.:mp[rrying ;rrlttrit!h[f;ltiirllf; p1t'rtrp ents et hcinre) wlth t ltrr rntt:trtill t:ollec'ting related or parallel kinds of survey infgrnrflriqrp,lri rrrr'fi r':r.rt:!.il poterntialfor differentialbias will exist due to the varilttiotrs i l i :!r'lI triil Moelc clln l: ;rlrl)(:irrto be particularly important when the survey questii:rnsan: :;i:llsilivir. l-lrttlrcl['two recent studies that systematically compared t(r :iilll$titllccuSCquestionsacrosstelephOneinterviews, faCe-to-face lTlipr:lll.',r..r.i rtnrl scll-administered(nonmail) questionnairesfound the strongest inlr:r'r,iew:.r, n,ir[:lrr:r: lirr urrclerreportingto exist for the telephonemode (Aquilino, 1994; licrrclrich and \/aughn, 1994)'. Aquilino suggeststhat this finding may be due tu rtspondent confidentiality concems. The telephone mode offers neither the .)l)portunity for building trust inherent in the face-to-face mode nor the response anonyrnity possible in the self-administered mode. Conclusion This review has been intended to illustrate that a total errol approach to survey design strives for a balance of design features that can maximize the validity of the survey data. Undue attention to reducing one kind of error will not ultimately prove beneficial if other forms of error ale thereby increased.For example, a researcher may attempt to reduce nonresponse by providing high incentives for survey partlcipation. But even if this strategy resuits -on"lary in a higher overall response rate,ll it also differentially boosts the representation of different subsetsof the population (without the application of compensatory weighring schemesduring data analysis) or if it also introduces the possibility of increased measurement bias (perhaps by making respondents to please the researchers),the total error will not be reduced, and the "uger will be ill advised. Comparable examples could be cited for other comstrategy binations of error as well. Researchpertinent to the prediction and contlol of suwey errors is accumulating at a very rapid pace. Careful attention to this growrng body of knowledge will help evaluators to increase substantially the precision of their data. On a larger scale, these methodological advances can contribute to the development of powerful new applications of surveys to evaluaiion settings. Note is oftenmadeto rhealtematetlpologythatidentifiesthetwo majorcategones 1 Reference Kasper, (described by Kish,1965.andAndersen, nonsamplingerrors of erroras sarnplingand (1989)modelto be conceptually clearer \979). I find Groves's Frankel,and Associates, terrnnonsamplingerrorisalooselydefinedcategorythatjoins because,ashepointsout,the error.Groves errorandmeasuremenl. asnonresponse elements suchverydisparate together errorsdue to model of survey is a comprehensive his framework als'onotesthat alihough it doesnot addresserrorsthat it is not a modelof totalsrweyerrorbecause designfeatures, errors. occurafterdatahavebeencollected,mostnotablycodingand processing 27 References Andersen,R., Kasper,J.,Frankel, M. R., and Associates.TotalSurveyError: Applicationsto Impro'teHealthSur-veys. San Francisco:Jossey-Bass,1979. Aquilino, W. S. "lntewiew Mode Effectsin Surveysof Drug and Alcohol Use:A Field Experiment." PublicOpinionQuarterly,1994, 58, 2IO-240. Biggar,R. J., and Melbye, M. 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