The 1.6 million P18-49 translates to 1.2 adults 18-49 rating.
via press note from TLC:
TLC's "Sarah Palin'S ALASKA" DELIVERS 4.96M TOTAL VIEWERS
The premiere of TLC's Sarah Palin's Alaska (Sun, Nov 14, 9-10 p.m. ET/PT) garnered a 3.5 HH rating and delivered 4.96M P2+, 1.8M P25-54, 1.1M W25-54, 1.6M P18-49, 4.7M P18+ and 3.9M P35+, according to Nielsen fast nationals.
Sarah Palin's Alaska is the #1 program launch in TLC history among HH and P2+ delivery, besting the What Not to Wear series launch in 2003, and posting triple digit rating and delivery gains vs. the prior six week prime average, and the 4Q10 to-date prime average.