NOTE 7—ACCRUED LIABILITIES AND DEFERRED REVENUE

Accrued liabilities are comprised of the following (in thousands):

Changes in the initial product warranty accrual and the expenses incurred under the Company’s initial and extended warranties for the three and six months ended June 30, 2016 and 2015 are included within accrued liabilities on the Consolidated Balance Sheets and were as follows (in thousands):

Deferred revenue is comprised of the following (in thousands):

In connection with the Company’s initiatives to measure and improve customer satisfaction and concurrent with the launch of WaterLase iPlus 2.0 in February 2015, the Company introduced its exclusive Practice Growth Guarantee, which is a program to assist with growth in the Company’s clients’ dental practices through training on a select number of clinical procedures and with billing and marketing support for dentists included. Consistent with the Company’s standard terms and conditions applicable to all of its products, the Practice Growth Guarantee does not give the customer the right to return purchased laser systems or receive a refund of any amount of the purchase price. However, the Practice Growth Guarantee does provide for additional training opportunities and certain billing and marketing support activities to the customer.  The Company has estimated additional deferred revenue related to the Practice Growth Guarantee for all WaterLase iPlus 2.0 system sales for the six months ending June 30, 2016 and June 30, 2015 to be approximately $132,000 and $94,000, respectively.