pre_text: Free cash flow Our non-GAAP financial measures also include free cash flow, which we define as cash provided by (used in) operating activities less the amount of property and equipment purchased. Management believes that information regarding free cash flow provides investors with an important perspective on the cash available to invest in our business and fund ongoing operations.
  
  However, our calculation of free cash flow may not be comparable to similar measures used by other companies. We believe these non-GAAP financial measures are helpful in understanding our past financial performance and our future results. Our non-GAAP financial measures are not meant to be considered in isolation or as a substitute for comparable GAAP measures and should be read only in conjunction with our consolidated financial statements prepared in accordance with GAAP.
  
  Our management regularly uses our supplemental non-GAAP financial measures internally to understand, manage and evaluate our business, and make operating decisions. These non-GAAP measures are among the primary factors management uses in planning for and forecasting future periods. Compensation of our executives is based in part on the performance of our business based on certain of these non-GAAP measures. We monitor the following non-GAAP financial measures:
table: For the year ended December 31,
table: (in thousands, except percentages and per share data) 2019 2018 2017
table: Non-GAAP gross profit $168,242 $163,376 $153,849
table: Non-GAAP gross margin 82.0% 84.6% 85.6%
table: Non-GAAP operating loss $(8,689) $(4,325) $(16,440)
table: Non-GAAP operating margin (4.2)% (2.2)% (9.1)%
table: Non-GAAP net loss $(9,460) $(4,548) $(16,594)
table: Non-GAAP net loss per share $(0.09) $(0.04) $(0.18)
table: Free cash flow $(3,924) $12,201 $(3,418)
question: Analyse this data from a financial earnings document. What was the average non-GAAP gross profit for the 3 year period from 2017 to 2019?