STOCK BASED COMPENSATION
Compensation expense associated with common stock, RSUs and restricted stock was recognized based on the fair value of the instruments, over the instruments’ vesting period. Stock based compensation expense decreased by $1.0 million for the year ended December 31, 2016 from the same period in 2015 primarily due to a one-time reversal of $2.0 million in stock compensation expense related to the 2015 and 2016 awards issued under the 2015 LTIP partially offset by the issuance and amortization of the 2016 grants under the 2015 LTIP. As of December 31, 2016 we do not currently believe it is probable that 50% of the awards issued in 2015 and 2016 will vest based on the related performance criteria and our assessment of the anticipated future performance applied to the performance criteria. The remaining 50% of awards expected to vest will continue to be expensed accordingly over the remaining applicable service periods. Stock based compensation expense decreased by $2.0 million for the year ended December 31, 2015 from the same period in 2014 primarily due to a lower number of shares issued for the 2015 grant under the 2015 LTIP plan as compared to the 2011 LTIP plan, which resulted in lower stock based compensation expense. Stock based compensation expense for the year ended December 31, 2015 also included a one-time charge of $0.3 million related to the departure of an executive.
The following table reflects the statement of income line items for stock based compensation expense for the periods stated:
Operating Expense Category
For the Year Ended December 31,
(Dollars in thousands)
2016
 
2015
 
2014
Cost of revenue (LTIP)
$
56

 
$
134

 
$
351

Research and development (LTIP)
52

 
86

 
90

Service, rental and maintenance (LTIP)
13

 
29

 
18

Selling and marketing (LTIP)
67

 
111

 
544

General and administrative (LTIP)
225

 
1,138

 
2,498

General and administrative (ESPP)
23

 

 

General and administrative (Board of Directors Restricted Stock)
418

 
370

 
337

Total stock based compensation
$
854

 
$
1,868

 
$
3,838

About Stock Compensation Disclosures

Stock-based compensation disclosures detail the equity awards granted to employees and executives — including stock options, restricted stock units (RSUs), and performance shares — along with the valuation methods and assumptions used to expense them. This section reveals the true cost of talent retention and the alignment between management incentives and shareholder interests.

Key signals: total unrecognized compensation expense and its expected recognition period signal future earnings headwinds from already-granted awards. For stock options, examine Black-Scholes assumptions — expected volatility, risk-free rate, and expected term — as understating any of these reduces reported compensation expense. Compare stock compensation expense as a percentage of revenue against peers to assess dilution cost. Watch vesting schedules for acceleration clauses tied to change-of-control events. Performance-based awards with undemanding targets may indicate weak governance. Add back stock compensation to operating cash flow to calculate a more conservative free cash flow figure.