We calculate an index of spot prices, which is visible in the web app Spot Index. The Spot Index has the following constituents:
The outliers (the lowest and the highest) are excluded and the remaining exchanges are averaged using equal values. This procedure applies even when data from any exchange above is not available. If there are 2 exchanges unavailable, the outliers are not excluded and the average is calculated using three remaining exchanges.
The QDX settlement index is calculated as 60-minute TWAP, i.e. it's an arithmetic average of the values of the spot index in the last 60 minutes (one value per minute).