Piper Jaffray analyst Christopher Raymond recommends using the selloff yesterday in shares of Biogen (BIIB) as a buying opportunity. The stock pulled back after management disclosed at a conference plans to up-size the Phase 3 aducanumab program by 510 patients total. While "this is a bit unnerving," particularly on the heels of another Phase 3 Alzheimer's failure for Merck's (MRK) verubecestat, the study remains blinded and variability on the primary endpoint can stem from a host of reasons, not all of them bad, Raymond tells investors in a research note titled "Aducanumab P3 Upsizing Doesn't Tell Us Much." The analyst does not think yesterday's news says much about the chances for an interim analysis or about aducanumab's efficacy profile. He keeps an Overweight rating on Biogen with a $407 price target.