One of the principal ways in which we make sense of the world is to group things and events into classes that share common characteristics. Human beings learn to do this intuitively in early childhood, and quickly develop an understanding of classes for commonly encountered objects and concepts which is shared by all those around them. However, for more specialised areas, the description of things or events in terms of classes tends to be a far less intuitive process that demands a carefully thought-out, explicitly articulated framework. Such classification frameworks make it much easier to compare information about entities and concepts, and to discern their similarities and differences.
Within the domain of public health, little conceptual work has been done to develop shared definitions, terminologies or classifications. As a result, we have limited ability to compare public health activity across jurisdictions and countries, or even to ascertain whether we share common notions of what constitutes 'public health'. This in turn hinders our ability to collect comparable, time-series data on expenditure, workforce, or performance, and to set and monitor benchmarks for these.
In Australia, the governments of the six states and two territories are the major providers of public health services, while the responsibility for funding these services is shared between the Australian (national) Government and state and territory governments . Local government (municipal and shire councils) also plays a role in delivering public health services, particularly in the areas of environmental health, urban planning, food safety and immunisation; this role varies among the states and territories.
The National Public Health Partnership, a body set up in 1996 to strengthen collaboration between the Australian Government and state and territory governments, adopted the following definition for public health:
the organised response by society to protect and promote health, and to prevent illness, injury and disability. The starting point for identifying public health issues, problems and priorities, and for designing and implementing interventions, is the population as a whole, or population sub-groups.
However, in Australia – as in other countries  – the term 'public health' is a source of confusion, because it is also often used to refer to health services provided by the state or otherwise paid for by taxpayers out of "the public purse", as opposed to services provided by the private sector, or paid for by individuals or nongovernmental health insurance or health maintenance funds. Some jurisdictions use the alternative term 'population health' to refer to the same domain, but this term is also poorly understood.
The lack of widespread understanding about what constitutes public health hampers efforts to advocate for more resources for the sector. If we as public health practitioners cannot clearly describe the activities of our sector, the resources expended by it, and its outcomes, it will remain difficult to convince the public, politicians and other decision-makers that greater investment is needed .
The lack of basic conceptual development within public health has been recognised internationally. An expert panel convened by the United States (US) Centres for Disease Control in 1999 identified the use of common definitions and comparable data sources as being among the most important issues for achieving the goal of quality improvement in public health through performance measurement . Five years later, lack of terminological and conceptual consensus was cited as obstructing even basic work in the area of public health finance in that country . In Australia, a 2002 project that set out to develop a key set of performance indicators for public health practice recommended, as a priority, the development of a common classification system that could be used for measuring expenditure as well as for organising performance measurement activities . A 2003 review of the financing of population health (defined as a subset of public health with a whole-of-population focus) in eight Organisation for Economic Co-operation and Development (OECD) countries, noted that comparability was hampered by differing definitions and categorisations of activity, lack of reliable data, and lack of uniformity in methods for extracting information .
Several conceptual models describing 'core' or 'essential' functions of public health exist, including the framework described in the Institute of Medicine's 1988 report on the status of public health in the US , the '10 essential public health services' proposed by an expert panel convened by the US Department of Health and Human Services , the Australian National Public Health Partnership's 'core functions for public health practice' , a set of core functions promulgated by the Chief Medical Officer in the UK , and another developed by the World Health Organization (WHO) in 1996  as well as 'essential public health functions' developed from a three-country study in WHO's Western Pacific Region in 2003 . The Pan American Health Organization developed 'essential public health functions' and public health 'roles' in a conceptual renewal of public health in 2002 , and revised these in 2007 . A list of 'essential functions' was recommended by the Canadian National Advisory Committee on Population Health in 2003  (See Additional file 1). These conceptual models, in particular the '10 essential public health services', have proved valuable for deriving performance indicators, standards and associated measurement instruments [17–23]. However, all are essentially 'flat' lists, or at best hierarchical taxonomies, which conflate discrete dimensions such as the purpose of public health activities, the health issues and problems addressed and the settings in which services are delivered, into single 'functions'. None presents a well-defined theoretical framework for multiple aspects of effective public health practice .
Some of the many standard classifications that are already in use in health fields address aspects of public health. For example, WHO's international classifications of diseases , functioning and disability , and external causes of injury  can be used to classify morbidity and mortality data in terms of diseases, disability and injury of interest to public health. The recently created OECD system of health accounts [28, 29] classifies health care in three dimensions for the purposes of international comparisons of health care spending: sources of funding; service providers; and functions of care (the goals or purposes of health care; e.g. disease prevention, health promotion). While these dimensions are clearly separate, the functional activity category of 'Prevention and public health services'  consists of a list of only six, non-exclusive, items (see Additional file 1).
A multidimensional approach was adopted by the Eastern Region Public Health Observatory in the United Kingdom (UK), for the construction of their Public Health Information Tagging Standard (PHITS). PHITS was developed to categorise and provide structure to information provided on websites, and to improve the efficiency of the retrieval of web-based public health resources. PHITS has seven dimensions: 'Person'; 'Time'; 'Place'; 'Determinants'; 'Morbidity and Mortality'; 'Services'; and 'Policy' . It has now been integrated into a UK National Public Health Language thesaurus .
PHITS was designed primarily to categorise web-based information resources, rather than as a multi-purpose classification for public health. Like other existing classifications, it does not capture all the important dimensions of public health, which include its functions, the methods and interventions used to achieve these, the health issues and determinants of health which public health activities address, the resources and infrastructure they use, and the settings in which the activities occur.
A multidimensional classification of public health that describes all these dimensions and their relationships, and adopts elements from existing classifications where appropriate, will serve multiple purposes. It will have utility for standardising the collection of information about public health programs, expenditure, workforce and performance. It will facilitate aggregate reporting and analysis of this information in ways that suit particular perspectives; for example, according to the health problem addressed, or the setting where public health activity occurred. This paper presents an initial version of such a multidimensional classification, and describes the process that we used to develop it .