# A Scalable Solution to Detect Behavior Changes of Elderly People Living Alone
## 1 Introduction
* Monitoring potential environmental hazards using dedicated equipment (e.g., gas leaks or fire). * Intrusion detection. Special intrusion detection systems are commercially available, equipped with passive infrared (PIR) motion detectors, glass break detectors, magnetic door contacts, and video surveillance cameras. * Sensory and cognitive assistance. This type of technology aims to compensate the sensory or memory loss, e.g., by using hearing aids and reminders about taking the medication. Some of these systems are also capable to provide verbal instructions on performing certain tasks, or orientation suggestions (see Ref. [17] for an extensive review on reminder systems). * Monitoring the level of social interactions by collecting data about the frequency and duration of the phone calls, the number of visitors received, and participation of the users in social activities ([18]). The assistive technology can go beyond the simple monitoring and, in certain applications, aims to facilitate social interactions of the monitored persons by including equipment that mediate the virtual meetings with friends or family, or the participation in group activities (e.g., through games, as in Ref. [19]). * Detection of emergencies (e.g., falls). A comprehensive review of the existing solutions for fall detection is available in Ref. [20]. * Functional monitoring. This research direction includes a wide variety of applications for monitoring the activities of daily life (ADL) and detection of behavior changes or abnormal activity patterns. Some of these applications are reviewed in Refs [21; 22; 23]. From the above list, of particular interest are the applications for ADL (long term) monitoring and detection of behavior changes, because many of the age-related pathologies have an insidious evolution over extended periods of time. Behavior changes, especially when they impact the daily activities of the elderly, usually indicate health issues, and may serve as clues for the early detection of a silent pathology. This is one of the reasons that explain the substantial number of articles published on this topic-for example, a simple search in the Web of Science for papers having the keyword \"behavior changes\" in the topic returns over 625,000 results (216,000 in the last 5 years). Despite this wealth of literature, it seems that there is still work to do until the results of the research in ADL recognition will be included in a practical solution applicable on a large scale ([24]). More specifically, [PERSON] et al. in Ref. [25] formulate a series of requirements for ADL monitoring systems, among which:  
* Economic Feasibility. These systems should be affordable for persons with lower socioeconomic status (SES). * Scalability. Monitoring systems should be designed so that they can be easily replicated in large series. * Unobtrusiveness. Ideally, the monitoring systems should be totally transparent for the users. * Continuity of sensing. The systems should be capable to collect and process data continuously over extended periods of time. * Usability. Such systems should be easy to install and require minimum maintenance to operate. * Adaptability. The monitoring systems should be easily adaptable to any individual regardless of her/his living environment. * A good trade-off between high sensitivity and a small number of false alarms. * Privacy and security. These systems should protect the privacy of the users and caregivers and have a clear and transparent policy regarding the usage of personal data. Another study ([26]) found that, from the perspective of the users' acceptance of the monitoring systems, the most principal factors that tend to hinder the adoption of the assistive technologies are the concerns about the loss of privacy and autonomy, along with the feelings of social isolation and stigma. Considering the above requirements and constraints, it appears that the sensors most suitable for ADL monitoring are the binary sensors (e.g., Passive Infrared-PIR motion detectors, magnetic contacts, water flow sensors, etc.).