Int. J. Environ. Res. Public Health 2009, 6 509 group (http://homecamp.org.uk) in London. An underlying assumption for many self-trackers is that data is an objective resource that can bring visibility, information and action to a situation quickly, and psychologically there may be an element of empowerment and control. Quantified self-tracking is being applied to a variety of life areas including time management, travel and social communications [20,21], as well as the health context, where the expanded definition of health is embraced as applications address both medical issues and general wellness objectives. 2.3.2. Quantified self-tracking definition Quantified self-tracking is the regular collection of any data that can be measured about the self such as biological, physical, behavioral or environmental information. Additional aspects may include the graphical display of the data and a feedback loop of introspection and self-experimentation. Health aspects that are not obviously quantitative such as mood can be recorded with qualitative words that can be stored as text or in a tag cloud, mapped to a quantitative scale, or ranked relative to other measures such as yesterday’s rating. Many health self-trackers are recording measurements daily or even more frequently (blood pressure for example). As with biomarker testing, health metric tracking was traditionally an expensive one-off process ordered by physicians for patients in response to specific medical risks. Two of the biggest applications in doctor-driven health metric tracking are cardiac monitoring and telemedicine (remote diagnosis) where implantable, worn or handheld devices transmit data wirelessly to medical professionals. Contemporary applications were featured at the 2008 USC Body Computing conference (http://www.usc.edu/schools/medicine/departments/medicine/divisions/cardiovascular/bcc) and the Unither Nanomedical and Telemedical Technology Conference (http://www.unithertechnology conference.com/conferenceagenda08.php). For example, Medtronic has an implantable pacemaker that transmits data using a portable monitor connected to a standard telephone line; physicians check the reading every three months [22]. A constant stream of on-demand pacemaker readings could be useful to both patients and physicians. HealthAnywhere (http://www.igeacare.com/HealthAnywhere/ Personal/home.htm) has an implantable monitoring system which provides early warnings of heart failure, transmitting data via cellular telephones and home computers. Mobile health monitoring company MedApps (http://www.medapps.net) is developing Bluetooth broadcasting monitors for common medical devices such as blood glucose meters, pacemakers and scales. 2.3.3. Categories and types of quantified self-tracking tools Focusing on the consumer self-tracking market, in addition to health social network-based self- tracking, there are a variety of other health monitoring websites and devices currently available. Some of these services and websites are listed in Table 2, they generally have some level of free services but are un-automated, meaning that users must input their own data. The websites may accept data via the Internet, SMS text messaging, instant messaging (IM), smartphone data applications, audio messages or other mechanisms.