Mark Suster Partner Upfront Ventures and former entrepreneur
“ Angel investors  [invest] at the highest risk because much less is proven in the business. They expect and need to earn a much higher return for the risk they’re taking.
 When a [Series] A investor gets involved, usually product has shipped, usually [there are] the first clients and some proof points, management team is better and solutions are more thought-out.
 [In] Series B [investors] are looking at metrics  like how many clients have [been] signed up,  costs of acquisition,  conversion rates,  LTV (lifetime value of customer),  churn, and they want to see data, facts and figures.
 Check sizes grow as the risk is decreased and usually coincides with the founders themselves needing more capital.” Mark Suster-This Week in Venture Capital #14 with Rick Smith, founder of CrossCut Ventures, Jul 15, 2010 @ 50-52 min into interview; http://www.bothsidesofthetable.com/2010/07/15/this-week-in-vc-with-rick-smith-of-crosscut-ventures/