Bloomberg Opinion columnist Barry Ritholtz speaks with David R. Kotok, who is co-founder, chairman and chief investment officer of Cumberland Advisors, which has $4 billion in assets under management. Kotok is also a director at the nonprofit Global Interdependence Center, which promotes free trade and international dialogue, and has authored or co-authored four books, including "From Bear to Bull with ETFs" and "Adventures in Muniland: A Guide to Municipal Bond Investing in the Post-Crisis Era."