"85% of engineers do not promote again past L4. For the 15% that’s left, 2–5 years later, they promote again. 85% of engineers do not promote again past that point. For the 15% that’s left, 2–5 years later, they promote again. And so on" -- From https://www.quora.com/profile/Dean-Jackson-3
有人通过LinkedIn 做了统计， 大约总共有950 L6, 300 L7, 110L8, 27 L9, 4 L10。 这个统计是2015年做的有些out of date，但是数量级应该差不多。那么L7& +总共有500人左右. 这些人都要求 significant impact and delivered mission critical projects. 非常有要求。
“Level 7- Principal Engineer / Level 8 - Distinguished Engineer: These are much higher impact roles. Such engineers have owned and delivered on very large mission-critical projects for Google (Think significant search engine/ads optimizations that have in a verifiable way, impacted Google positively). They are responsible for initiating, driving, technically designing, implementing and productionizing high impact ideas at Google with the help of teams if needed.”
