Key Performance Indicators
In addition to traditional financial metrics, such as revenue and revenue growth trends, we monitor several other key performance indicators to help us evaluate growth trends, establish budgets, measure the effectiveness of our sales and marketing efforts and assess operational efficiencies. The key performance indicators that we monitor are as follows:
(1) Adjusted EBITDA and revenue constant currency growth rate are non-GAAP financial measures. For a reconciliation of Adjusted EBITDA and revenue constant currency growth rate to the nearest comparable GAAP measures, see Item 6. “Selected Financial Data.” (2) Reflects the customer count on the last day of the period rounded to the nearest hundred customers.
Revenue constant currency growth rate. We believe revenue constant currency growth rate is a key indicator of our operating results. We calculate revenue constant currency growth rate by translating revenue from entities reporting in foreign currencies into U.S. dollars using the comparable foreign currency exchange rates from the prior fiscal periods.
For further explanation of the uses and limitations of this non-GAAP measure and a reconciliation of our revenue constant currency growth rate to revenue, as reported, the most directly comparable U.S. GAAP measure, see Item 6. “Selected Financial Data.” As our total revenue grew over the past three years, our revenue constant currency growth rate has decreased over the same period, as the incremental growth from period to period represented a smaller percentage of total revenue as compared to the prior period.
As our total revenue grows, we expect our constant currency growth rate will decline as the incremental growth from period to period is expected to represent a smaller percentage of total revenue as compared to the prior period.
Revenue retention rate. We believe that our ability to retain customers is an indicator of the stability of our revenue base and the long-term value of our customer relationships. Our revenue retention rate is driven by our customer renewals and upsells. We calculate our revenue retention rate by annualizing constant currency revenue recorded on the last day of the measurement period for only those customers in place throughout the entire measurement period.
We include add-on, or upsell, revenue from additional employees and services purchased by existing customers. We divide the result by revenue on a constant currency basis on the first day of the measurement period for all customers in place at the beginning of the measurement period. The measurement period is the trailing twelve months.
The revenue on a constant currency basis is based on the average exchange rates in effect during the respective period. Our revenue retention rate in fiscal 2019 was relatively consistent with fiscal 2018. We expect our revenue retention rate to remain relatively consistent for fiscal 2020.
Total customers. We believe the total number of customers is a key indicator of our financial success and future revenue potential. We define a customer as an entity with an active subscription contract as of the measurement date. A customer is typically a parent company or, in a few cases, a significant subsidiary that works with us directly. We expect to continue to grow our customer base through the addition of new customers in each of our markets.
Gross profit percentage. Gross profit percentage is calculated as gross profit divided by revenue. Our gross profit percentage has been relatively consistent over the past three years; however, it has fluctuated and will continue to fluctuate on a quarterly basis due to timing of the addition of hardware and employees to serve our growing customer base. More recently, gross profit has also included amortization of intangible assets related to acquired businesses.
We provide our services in each of the regions in which we operate. Costs related to supporting and hosting our product offerings and delivering our services are incurred in the region in which the related revenue is recognized. As a result, our gross profit percentage in actual terms is consistent with gross profit on a constant currency basis.
Adjusted EBITDA. We believe that Adjusted EBITDA is a key indicator of our operating results. We define Adjusted EBITDA as net (loss) income, adjusted to exclude: depreciation, amortization, disposals and impairment of long-lived assets, acquisition-related gains and expenses, litigation-related expenses, share-based compensation expense, restructuring expense, interest income and interest expense, the provision for income taxes and foreign exchange income (expense).
Adjusted EBITDA also includes rent paid in the period related to locations that are accounted for as build-to-suit facilities. For further explanation of the uses and limitations of this non-GAAP measure and a reconciliation of our Adjusted EBITDA to the most directly comparable U.S. GAAP measure, net (loss) income, see Item 6. “Selected Financial Data.”
We expect that our Adjusted EBITDA will continue to increase; however, we expect that our operating expenses will also increase in absolute dollars as we focus on expanding our sales and marketing teams and growing our research and development capabilities.
Table:
 |  | Year Ended March 31,  | 
 | 2019 | 2018 | 2017
 |  | (dollars in thousands)  | 
Revenue constant currency growth rate (1) | 32% | 38% | 39%
Revenue retention rate | 111% | 110% | 111%
Total customers (2) | 34,400 | 30,400 | 26,400
Gross profit percentage   | 73% | 73% | 73%
Adjusted EBITDA (1) | $54,008 | $25,752 | $12,457


Question: In which year was Adjusted EBITDA less than 20,000 thousands?
Answer: 2017
Calculation: locate and analyze Adjusted EBITDA in row 8