Shake Shack (SHAK) and Spark Therapeutics (ONCE) join a group of 15 IPOs since 2013 that have popped 100% or more on the first day. 12 of the 13 other IPOs fell after the first day, though most of them still trade above the original offer price. The list includes three other fast casual restaurant concepts and four other small health care companies. Can Shake Shack and Spark maintain their valuations?
