The phrase "previous estimates" refers to the guesses or calculations made before based on the available information or data. It means the earlier assumptions or predictions that have been made in the past. Full definition
This suggests that it has fallen far short of previous estimates of success. (sciencedaily.com)
A 2014 study showed that we can distinguish at least 1 trillion different odors — up from previous estimates of a mere 10,000. (discovermagazine.com)
The new study suggests that the impact may have released around three times as much sulfur and much less carbon dioxide compared with previous estimates from 20 years ago. (sciencenews.org)