In this installment of My Favorite Number, the World Bank Group's Douglas Pearce says he's most intrigued by the number 2 billion. That staggering sum represents the "unbanked"; people without access to transaction accounts. That lack of access is especially a problem for the poor, for women, and for people in rural areas. But a new focus on leveraging data and digital technology is transforming the business model for providing financial services for low income individuals and for small enterprises, and is opening up opportunities which can help reduce poverty for millions.