The thousands of individual population trends are brought together to calculate the average percentage change in population sizes using an index (below). The percentage doesn't represent the number of individual animals lost; instead, it reflects the average change in animal population sizes.
The 2020 global Living Planet Index shows an average 68% fall in monitored vertebrate species populations between 1970 and 2016.
The data is gathered from almost 4,000 sources, using increasingly sophisticated technology such as audio devices to monitor insect sounds; drones and satellite tagging to track populations on the move; and even blockchain technology to track the impact of harvesting on wild populations. (Cook, B. 2018).