Greatness combines multiple measures from Crucible into a single-number that summarizes your greatness in the Crucible. It is calibrated so that a really good value is 1.0.
The general formula is: Greatness = Skill x Experience.
Skill calculationOverall PvP
0.4*kd + 0.2*win_loss + 0.1*spree/10 + 0.1*max_kills/20
0.1*kd/0.5 + 0.1*win_loss/0.4
Experience is given a number between 0.05 and 1, and depends on the number of matches you have played. This way, if you have played just one PvP match, you can't truly be great.
<20 matches: 0.05
20-49 matches: 0.8
>=50 matches: 1.0
Using the above definitions, greatness is calculated as:
crucible_skill*crucible_experience + pvp_skill*pvp_experience