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Stock-Based Compensation (Tables)
6 Months Ended
Jun. 30, 2019
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Stock-based compensation expense
Stock-based compensation expense included in the consolidated statements of operations was as follows:
 
Three Months Ended June 30,
 
Six Months Ended June 30,
(in thousands)
2019
 
2018
 
2019
 
2018
Cost of revenue
$
742

 
$
114

 
$
1,394

 
$
191

Sales and marketing
4,215

 
675

 
7,581

 
1,277

Research and development
2,441

 
640

 
4,471

 
1,167

General and administrative
3,975

 
1,595

 
7,246

 
2,788

Total stock-based compensation expense
$
11,373

 
$
3,024

 
$
20,692

 
$
5,423


Stock option activity
A summary of our stock option activity is presented below:
(in thousands, except for per share data and years)
Number
of Shares
 
Weighted
Average
Exercise Price
 
Weighted-Average Remaining Contractual Term (in years)
 

Aggregate Intrinsic Value
Outstanding at December 31, 2018
19,219

 
$
7.78

 
8.0
 
$
277,114

Granted

 

 

 


Exercised
(3,242)

 
3.93

 

 


Forfeited/canceled
(1,085)

 
10.11

 

 


Outstanding at June 30, 2019
14,892

 
8.45

 
7.7
 
299,201

Exercisable at June 30, 2019
6,213

 
5.72

 
6.9
 
141,797


Summary of restricted stock and restricted stock units
A summary of our restricted stock and restricted stock units activity is presented below:
 
Restricted Stock
 
Restricted Stock Units
(in thousands, except for per share data)
Number
of Shares
 
Weighted
Average
Grant Date Fair Value
 
Number
of Shares
 
Weighted
Average
Grant Date Fair Value
Unvested balance at December 31, 2018
890

 
$
4.25

 
1,129

 
$
18.90

Granted

 

 
2,003

 
29.10

Vested
(198
)
 
4.25

 
(1
)
 
23.00

Forfeited

 

 
(144)

 
20.84

Unvested balance at June 30, 2019
692

 
4.25

 
2,987

 
25.65


Stock option valuation assumptions
The fair value of the 2018 ESPP purchase rights for the initial and second offering periods was estimated on the grant date using a Black-Scholes option-pricing model and the following assumptions:
Expected term (in years)
0.5 — 2.1
Expected volatility
31.9% — 44.6%
Risk-free interest rate
2.3% — 2.7%
Expected dividend yield