pre_text: Free cash flow We define free cash flow as net cash provided by (used in) operating activities less purchase of property and equipment. We consider free cash flow to be a liquidity measure that provides useful information to management and investors about the amount of cash generated by the business that can possibly be used for investing in our business and strengthening the balance sheet, but it is not intended to represent the residual cash flow available for discretionary expenditures. Free cash flow is not prepared in accordance with U.S. GAAP, and should not be considered in isolation of, or as an alternative to, measures prepared in accordance with U.S. GAAP. The following table presents a reconciliation of net cash provided by (used in) operating activities to free cash flow: (1) Our adoption of ASU 2016-09 on January 1, 2017 resulted in excess tax benefits for share-based payments recorded as a reduction of income tax expense and reflected within operating cash flows, rather than recorded within equity and reflected within financing cash flows. We elected to adopt this new standard retrospectively, which impacted the presentation for all periods prior to the adoption date.
table: Year Ended December 31,
table: 2019 2018 2017 2016 2015
table: (in thousands)
table: Net cash provided by (used in) operating activities (1) $(156,832) $113,207 $64,241 $138,720 $141,257
table: Purchase of property and equipment (36,531) (52,880) (89,160) (78,640) (30,566)
table: Free cash flow $(193,363) $60,327 $(24,919) $60,080 $110,691
table: Net cash provided by (used in) investing activities $25,761 $17,496 $(28,718) $(392,666) $(170,027)
table: Net cash provided by (used in) financing activities (1) $(8,406) $1,287 $4,635 $19,794 $368,953
question: Analyse this data from a financial earnings document. What is the average net cash provided by operating activities for 2016 and 2017?