S-I/A and marketing efforts. We measure the effectiveness of our spending by evaluating the "payback period? which we view as the number of quarters it takes for a quarterly cohort of sellers' cumulative transaction revenue net of transaction costs to surpass our sales and marketing spending in the quarter in which we acquired that cohort. We define a quarterly cohort of sellers as the group of sellers that are approved to accept card payments with Square in a given quarter. On average, our payback period has been four to five quarters. Revenue from our sellers has grown consistently over time, resulting in strong dollar-based retention rates. Transaction revenue net of transaction costs for each of our 18 quarterly seller cohorts (dating back to the second quarter of 2010) has grown year over year in every quarter since the first quarter of 2012. Over the past four quarters, retention of transaction revenue net of transaction costs for our cohorts has, on average, exceeded 110% year over year. 2 Table of Contents The size and strength of our payments and POS business have allowed us to develop a deep understanding of our sellers' business performance and to build a cohesive commerce ecosystem. As such, we are well positioned to provide financial services and marketing services to sellers efficiently and intelligently. For example, one of our financial services, Square Capital. uses our deep understanding of our sellers' businesses to proactively underwrite and extend cash advances to them. Although Square Capital is still in its early stages, we have already advanced over $300 million across over 50.000 advances since launching it in May 2014. Square Capital demonstrates how our services can rapidly reach significant scale through a combination of strong demand and our direct, ongoing interactions with our sellers. Although Square Capital currently does not contribute a significant amount of revenue to our business relative to our payments and PO