W3C - The supply index

The supply index should always be 100.
A lower value means that the country does not have what it needs to function and it creates a negative influence on people in the country.

The supply index can be quickly reduced to much lower values if the country suffers shortages.

On the trade page, look into the option that allows you to purchase products the country needs.
The page shows, for every single product, how much you have in stock and how much is used each game month.

check for numbers in red.
Several products showing shortages (red) will diminish you supply index.

The supply index is one of the welfare factors. The welfare index is not a simple average of indexes. It is the minimum of several values that depend on various indexes, including the supply index.

Once the supply index goes under, it can drag down the entire welfare index and with it, the entire industry of the country.