$2,415,000 of the increase, as a result of various grants made in July 2015 and December 2015 (including accelerated vesting in December 2015 of prior grants) in connection with agreements to defer payments. The remaining increase was a result of the changes in the various components of our operating expenses, which are described below.
Cost of revenue
We had cost of revenue of $2,847,000 for the year ended December 31, 2015, an increase of $2,155,000, or 311.4%, from $692,000 for 2014. The primary reason for the increase was $1,768,000 of additional start-up and operational costs in 2015 associated with the provision of WellChec™ services by third-party vendors and employees who oversee manufacturing operations and fulfillment of both services and products. The total increase of $2,155,000 is composed of the following categories. A portion of the increase is due to the fact that aggregate depreciation of our vascular testing products for lease increased $77,000, or 39.7%, in 2015 compared to 2014 as there was a 43.9% increase in the number of installed units in the field incurring monthly depreciation charges corresponding to the 43.9% increase in number of installed units in the field generating monthly revenue, partially offset by a decrease in average depreciation per unit per month of 2.9%. Other reasons for the $2,155,000 increase include $1,415,000 of headcount related expenses, $356,000 of WellChec™ event expenses, $109,000 total depreciation (including increase in depreciation of assets for lease of $77,000 as previously described, and additional depreciation of $32,000 for capital equipment purchased to support WellChec™), $107,000 for freight and shipping, $66,000 higher cost of units that were retired in 2015 compared to 2014, $63,000 for other expenses, and $39,000 for building lease.
Engineering and product development expense
We had engineering and product development expense of $1,436,000 for the year ended December 31, 2015, an increase of $323,000, or 29.0%, compared to $1,113,000 in 2014. The increase was primarily due to higher salaries of $737,000, higher stock-based compensation expense of $100,000, higher clinical studies cost of $48,000, higher other expenses of $25,000, and higher travel costs and other expenses of $15,000 partially offset by lower costs for new product development of $602,000.
Sales and marketing expense
We had sales and marketing expense of $6,266,000 for the year ended December 31, 2015, an increase of $2,543,000, or 68.3%, compared to $3,723,000 in 2014. The increase was primarily due to higher stock-based compensation expense of $1,166,000, higher salary expense of $865,000 associated with having an expanded sales team as compared to the prior period, higher travel expenses of $347,000, higher rent of $160,000, higher other expenses of $101,000 and higher trade show expense of $68,000, partially offset by $164,000 of lower sales commissions.
General and administrative expense
We had general and administrative expense of $4,871,000 for the year ended December 31, 2015, an increase of $2,423,000, or 99.0%, compared to $2,448,000 in the same period in 2014. The increase was primarily due to higher stock-based compensation expense of $1,147,000, higher salaries and fees for employees, directors, and consultants of $589,000, medical device excise tax, state and local tax, audit and tax preparation expenses of $227,000, an increase in uncollectible accounts of $143,000, higher merchant fees and other expenses of $126,000, higher insurance premiums of $105,000, added costs associated with being publicly traded company of $53,000, higher patent and legal expenses of $26,000, higher technical support costs of $4,000, and higher travel costs of $3,000.
Other expense
We had other expense of $82,000 for 2015, a decrease of $92,000, or 52.9%, compared to $174,000 in 2014. The decrease was primarily due to lower interest expense of $101,000, partially offset by an increase in other expense of $9,000.