109/264 Premium partner services are priced on a per­subscriber rate in a negotiated agreement and may include minimum guarantees for the number of subscriptions that will be purchased from us. Revenue for our Premium segment is a function of the number of Premium Subscribers who use our Premium Service. As of December 31, 2016 and 2017, we had approximately 48 million and 71 million Premium Subscribers, respectively. New Premium Subscribers primarily are sourced from the conversion of our Ad­Supported Users to Premium Subscribers. Through both our online platform and external marketing efforts, we engage our Ad­Supported Users by highlighting key features that encourage conversion to our subscription offerings. These efforts include product links, campaigns targeting existing users, and performance marketing across leading social media platforms. Additionally, new subscriber growth also is driven by the success of converting users from our bi­annual trial programs to full­time Premium Subscribers. These trial campaigns typically offer our Premium Service free or at a discounted price for a period of time and accounted for approximately 23% and 20% of total gross added Premium Subscribers for the years ended December 31, 2016 and 2017, respectively. Premium Subscribers in Europe Premium Subscribers in North America Premium Subscribers in Latin America Premium Subscribers in Rest of World With engagement increasing, Premium Churn has continued to trend lower. In the fourth quarter of 2017, Premium Churn was 5.1%, down from 6.0% in the fourth quarter of 2016 and down from 7.5% in the fourth quarter of 2015. As our User base matures, and with the growth in higher retention products such as our Family Plan and Student Plan, we believe Premium Churn will continue to trend lower over time. The trend toward lower Premium Churn has had a positive impact on the lifetime value of a Premium Subscriber (“LTV”) and the ratio of LTV to the average cost of acquiring a Premium Subscriber (“SAC”). Excluding expenses associated with the Ad­Supported segment, LTV/SAC was 3.6x in the fourth quarter of 2017. Here, LTV is calculated by dividing one by the Premium Churn rate for the fourth quarter of 2017 multiplied by the Premium ARPU for the fourth quarter of 2017 and by gross margin for the Premium segment for the fourth quarter of 2017; and SAC is calculated by dividing sales and marketing expense for the Premium segment for the fourth quarter of 2017 by gross Premium Subscriber additions for the fourth quarter of 2017. Including expenses associated with the Ad­Supported segment, LTV/SAC was 2.7x. Here LTV is calculated by dividing one by the Premium Churn rate for 102