Although numerous studies have identified, classified, and measured behaviors among individuals in congregate settings and among those with cognitive impairment (Burgio et al., 2002; Cohen-Mansfield, Marx, & Rosenthal, 1989; Logsdon & Teri, 1997; Rose & Pruchno, 1999; Talerico & Evans, 2000; Teresi et al., 1997), practically none have focused on resident-to-resident elder mistreatment (R-REM). To our knowledge, there is no psychometrically developed measure of R-REM extant. Behaviors directed by residents against other residents are potentially serious because they can be sudden and unexpected, leaving the recipients fearful in anticipation of the next occurrence. The prevalence of R-REM has not been demonstrated fully; however, Lachs, Bachman, Williams, and O’Leary (2007) found that almost 6% of community residents had police contact after being placed in a long-term care institution, the majority of which were due to R-REM. A recent study of combative behaviors of residents (Morgan et al., 2012), although not focused on R-REM, used diaries and questionnaires completed by frontline staff to document incidents of aggression directed toward them. The majority of staff reported occurrence of incidents within the past month, providing evidence regarding the ubiquity of such aggressive behaviors. Although there are many factors that contribute to resident-to-resident aggression, chief among them is the commingling of residents with a mixture of cognitive impairment, comorbidity, and differing lifestyles and personalities in small places (Teresi, Holmes, & Monaco, 1993). For example, in one of the few studies of R-REM extant, Shinoda-Tagawa and colleagues (2004) found that cognitive impairment, aggressive behavior, and physical independence were associated with physical abuse of recipients. Measurement in the study by Shinoda-Tagawa and colleagues (2004) was from reports to the Department of Health and abstraction of police reports. Most recently, Castle (2012), building on the conceptual work of Lachs and Pillemer (2004) and Stiles, Koren, and Walsh (2002), assessed resident-to-resident abuse using a mailed questionnaire completed by certified nursing assistants (CNAs), with 35 items measuring five domains: verbal, physical, psychological, sexual abuse, and material exploitation (taking other peoples belongings). Although the study was large, with facilities sampled from 10 states, the authors discuss that more refined measures focused on specific residents rather than aggregated reports are needed to determine prevalence and better understand the mechanisms of R-REM. The first study of abuse of patients in nursing homes (Pillemer & Moore, 1989) resulted in a taxonomy of abuse, based on earlier work in the community (Pillemer & Finkelhor, 1988) using an instrument designed to measure forms of family violence, on which the instrument derived for this study was partially based. The methodology has been replicated in several other countries and has produced remarkably stable estimates of self-reported community elder abuse (Lachs & Pillemer, 2004). There is no “gold standard” in violence research because researchers or anyone outside the violent dyad rarely witnesses actual episodes. Self-report is widely accepted as the primary methodology for case finding in violence research, and this method has been used in this study. Given the high prevalence of cognitive impairment in long-term care settings, staff reports are critical in the assessment of R-REM. Aims The purpose of this article is to describe the quantitative development of a measure of staff-reported R-REM (R-REM-staff version; R-REM-S). The qualitative development is described elsewhere (Ramirez et al., in press). Conceptual Model The conceptual model is derived from earlier work by Pillemer and Moore (1989) and by qualitative research using focus groups (Rosen et al., 2008). In the latter study, R-REM was categorized as physical, verbal, and sexual and 35 types were identified within these classifications. Later, Pillemer and colleagues (2011) characterized R-REM in terms of typologies, identifying five categories as follows: invasion of privacy, roommate altercations, hostile interpersonal interactions, unprovoked actions, and inappropriate sexual behaviors. Qualitative work (Ramirez et al., in press) identified four broad domains of R-REM: verbal, physical, sexual, and other (e.g., wandering into others’ rooms). These domains form the basis for the measure presented here. For the purpose of this study, resident-to-resident mistreatment is defined as negative and aggressive physical, sexual, or verbal interactions between residents in long-term care settings, which in a community setting would likely be considered as unwelcome. The general approach taken was derived from a long-standing and well-established measurement strategy in the field of interpersonal aggression. The best known of the instruments based on this strategy is the Conflict Tactics Scale (CTS; Straus, Hamby, Boney-McCoy, & Sugarman, 1996). Measures based on the CTS model have been successfully used with nursing home staff (Pillemer & Moore, 1989). The CTS conforms to a major finding regarding effective measurement of mistreatment, that is, violence measures must be specific. Behaviorally specific questions have a number of advantages: They require fewer definitions of terminology (as would a general question such as “Have you ever been abused?”), they are less subject to a respondent’s interpretation of terms, and they are less dependent on educational level. Most important, behavioral specificity reduces the effects of potential personal and cultural differences in interpretation of terms such as “abuse” or “neglect” (Acierno, 2003). A further strength of the CTS approach is separating the focus on behaviors from outcomes. The approach is consistent with legal definitions, which negatively sanction assault regardless of whether it results in serious injury. R-REM may also be serious but does not cause obvious physical harm. Shinoda-Tagawa and colleagues (2004) included only individuals who had suffered a documented injury in their study. This rule is likely to exclude individuals who may be psychologically distressed by an incident or who may have been injured in ways that are not obvious (Herbert & Bradshaw, 2004). Thus, the present measure moves beyond the focus only on R-REM that has resulted in injury and includes a wide range of actions, regardless of outcome. One central feature of R-REM may separate it from nearly every other form of interpersonal violence—it occurs in an environment that is both a residence as well as a health care facility. Accordingly, many health care providers are potentially “stationed” to detect R-REM either directly or indirectly. To not make systematic use of these observers would be to lose an opportunity for more comprehensive case finding and description. Thus, this study used staff reports via a staff version of the R-REM measure, after brief in-service training that is part of customary periodic facility education (e.g., infection control). Although few systematic studies of staff reporting of R-REM exist, it is noted that nursing home staff have been “violence and conflict reporters” in other research studies not involving R-REM. Remarkably, nursing home staff members have actually reported their own acts of mistreatment (anonymously) toward residents as well as witnessed acts of abuse by other staff (Pillemer & Moore, 1989). These studies would suggest that nursing home staff could be reliable informants for detecting and describing R-REM, particularly because it is not as personally stigmatizing as self-promulgated abuse.