Earlier quoted context omitted.
What accuracy do you think humans have? If I give you 1000 handwritten numbers, do you think you'll make less than 10 mistakes?
The forms are in open access, anyone can fill one form, so they surely did consider variations in accuracy (if not plain sabotage). They are probably submitting the same data to multiple people and the cross-checking the submissions. Also, the totals help to filter out submissions with mistakes. One can also add bound checking for individual values (misplaced decimal point, extra digits etc.). I would love to see how…
This is exactly how zooniverse works. The first thing you do is go through a few known pieces of data to verify you know how to identify things correctly (more important for galaxy types, etc).
As you start labeling data everything has to reach a consensus. I'm not sure exactly how it works, but it does have multiple people verify each piece of data.