Item 6. Selected Financial Data

You should read the following selected financial data in conjunction with "Management's Discussion and Analysis of Financial Condition and Results of Operations" and our financial statements and the related notes appearing in "Financial Statements and Supplementary Data."

We were incorporated in May 2014 and, pursuant to a series of pre-IPO organizational transactions, became a holding company whose principal asset is a controlling equity interest in Desert Newco. We are the sole managing member of Desert Newco, and as a result, we consolidate its financial results and report non-controlling interests representing the economic interests held by its other members. Because our pre-IPO organizational transactions were considered transactions between entities under common control, the financial statements for 2015 have been adjusted to combine the previously separate entities for presentation purposes.

The statements of operations data for the years ended December 31, 2019, 2018 and 2017, and the balance sheets data as of December 31, 2019 and 2018, are derived from our audited financial statements and the related notes appearing in "Financial Statements and Supplementary Data." The statements of operations data for the years ended December 31, 2016 and 2015, and the balance sheets data as of December 31, 2017, 2016 and 2015, are derived from our audited financial statements not included in this Form 10-K. Our historical results are not necessarily indicative of the results to be expected in any future period.

(2) Amounts for 2015 have been retrospectively adjusted to give effect to our pre-IPO organizational transactions.

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(1) Total debt includes long-term debt, unamortized original issue discount and unamortized debt issuance costs.

Key Metrics

In addition to our results determined in accordance with GAAP, we believe the following operating metrics are useful as supplements in evaluating our ongoing operational performance and help provide an enhanced understanding of our business:

Total bookings. Total bookings represents cash receipts from the sale of products to customers in a given period adjusted for products where we recognize revenue on a net basis and without giving effect to certain adjustments, primarily net refunds granted in the period. Total bookings provides valuable insight into the sales of our products and the performance of our business since we typically collect payment at the time of sale and recognize revenue ratably over the term of our customer contracts. We report total bookings without giving effect to refunds granted in the period because refunds often occur in periods different from the period of sale for reasons unrelated to the marketing efforts leading to the initial sale. Accordingly, by excluding net refunds, we believe total bookings reflects the effectiveness of our sales efforts in a given period.

Total customers. We define a customer as an individual or entity, as of the end of a period, having an account with one or more paid product subscriptions. A single user may be counted as a customer more than once if the user maintains paid subscriptions in multiple accounts. Total customers is one way we measure the scale of our business and is an important part of our ability to increase our revenue base.

Average revenue per user (ARPU). We calculate ARPU as total revenue during the preceding 12 month period divided by the average of the number of total customers at the beginning and end of the period. ARPU provides insight into our ability to sell additional products to customers, though the impact to date has been muted due to our continued growth in total customers.

Reconciliation of Bookings

The following table reconciles total bookings to total revenue, its most directly comparable GAAP financial measure:

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(1) Change in deferred revenue also includes the impact of realized gains or losses from the hedging of bookings in foreign currencies.