**Availability of data**

Only public data sources are used for the SSI. And of course we use the most recent data available. Nevertheless, for the SSI-2016 most data are of 2014, some are more recent, some are of earlier years.

Particularly for this update we were confronted with a large number of recalculated data by the official databases. We have followed these updates and ensured that this has been done for all countries and for historic data also. It caused some pretty large changes in the data. Thus one cannot just compare the results of this edition of the SSI with previous ones. However, as said, one can compare the data presented in this edition over time.

** Reliability of data**

The reliability of data remains a serious concern. One is inclined to assume published figures to be correct and reliable. However, this is certainly way too optimistic. We only have to refer to the paragraph above. Particularly when producing time series one is confronted with many irregularities and impossibilities in the data. This problem may decrease over time, since the importance of sound statistical data is now generally recognized.

** Aggregation**

Opinions concerning aggregation vary enormously. For some it is an absolute ‘don’t’, others simply do it. In view of the objectives of the SSI – among others to show at a glance the level of sustainability of a country – we have aggregated the scores of the indicators into scores for the three wellbeing dimensions.

For the aggregations we have used the geometric average. The arithmetic average, offers the possibility of compensation: low scores for one indicator can be compensated by high scores for a different indicator. Since sustainability, i.e. strong sustainability, doesn’t allow compensation, it is best to use the geometric average, which suppresses this compensation.

Following the recommendations of Joint Research Centre (JRC) of the European Commission in its audit of the SSI, we haven’t aggregated the dimension levels into one single figure for the overall index. The reason being the negative correlation between Human and Environmental Wellbeing.

For those who object to aggregations and are only interested in the underlying figures, we present all available data. Thus the user may make its own choice: focus on the aggregated scores or on the underlying figures. Or on both.

**Calculation methodology**

For lack of a scientific basis for the attribution of different weights to the indicators, every indicator has received the same weight for the aggregation into dimensions.

All totals, be it for the world as a whole, per income class or per region, are weighted for population size. This means that an inhabitant of Iceland has the same weight as an inhabitant of China. In the data tables we have also presented the averages per country. Thus one can notice the impact of weighting per person or per country.

**Calculation formulas**

If you are interested in the formulas used to calculate the indicators, you can have a look at the file you can download here. For each indicator the formula is shown, in which F(X) is the indicator score and X the value of the raw data. In addition the range of validity is indicated. The graphs show the resulting scores for the 154 countries included in the SSI, visualizing the range of Indicator scores and the range of raw data values.