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Threshold 21 Model
Threshold 21 iSDG Model
MEG Model
M3 Model

The T21 Starting Framework

The development of each national T21 model starts with the implementation of a Starting Framework, which is subsequently customized to capture the peculiar issues of the country being analyzed. The T21 Starting Framework is a generic structure that represents development mechanisms that can be found in most developing and industrialized countries. It covers a broad range of issues that countries the world over face on the path to sustainable development, for example, poverty, environmental degradation, education, healthcare, economic growth, and demographic shifts. In order words, the Starting Framework is designed to cover the most common long-term issues countries encounter in the development process.

The T21 SF is a relatively large size model, comprising of more than a thousand equations, about 60 stock variables, and several thousands feedback loops. Given the size and the level of complexity of the model, its structure has been reorganized into smaller logical units, called modules.

The T21 SF is composed of 37 modules. A module is a piece of the T21 model whose internal mechanisms can be understood in isolation from the rest of the model. The size of a module is determined based on consideration of the amount of information a user can take in at once, and the standard size of computer monitors. All modules fit in a 1024x768 screen.

T21’s modules are grouped into 18 sectors: 6 social sectors, 6 economic sectors, and 6 environmental sectors. A sector is a group of one or more modules related by their functional scope. For example, the water sector groups the water demand and water supply modules; and the education sector groups the primary education and secondary education modules.

Society, Economy and Environment are known as the three spheres of T21. All sectors in the T21 SF belong to one of the three spheres, depending on the type of issue they are designed to address (see Figure 1). Modules are built to be in continuous interaction with other modules in the same sector, across sectors, and across spheres.

Figure 1: Spheres and sectors


The T21 SF should not be considered a rigid framework, but rather a starting point for creating a fully customized national development planning model. In most cases, additional modules have to be introduced to capture the particular reality of a country. Similarly, some modules may need to be eliminated during the customization process because they are not relevant to a country’s peculiar situation.

Examples of modules that are very often added to the initial structure are the indicators modules. These are modules created to calculate or organize specific indicators, including but not limited to Millennium Development Goals (MDG), Human Development Index (HDI), and Gender Development Index (GDI). These and many others are ready, on-the-shelf modules to be linked to the Starting Framework as needed.

The structure of the individual modules is based on well accepted work in the field, “translated” in stock and flow language by MI modelers and integrated with ad-hoc research. A distinctive characteristic of T21 is the way the various modules and sectors are linked together to form a complex network of feedback loops that determine the model’s behavior. In Figure 2, the three Spheres are exploded to highlight the several cross-sector linkages and the resulting feedback loops. Each of these loops has a specific and often fundamental role in driving or limiting a country’s development. It is essential for development to be fully sustainable that policy makers can identify the key forces at work in the system and alter their relative strength as desired. T21 is a guiding light for policymaking in these highly complex systems.


Figure 2: Example of linkages between sectors

Figure 2: Example of linkages between sectors