Quarterly report pursuant to Section 13 or 15(d)

Quarterly report pursuant to Section 13 or 15(d)

Equity

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Equity
3 Months Ended
Mar. 31, 2019
Equity [Abstract]  
Equity Equity
We recorded approximately $2.1 million and $1.6 million in stock based compensation expense for the three month periods ended March 31, 2019 and 2018, respectively.
A summary of the stock option activity under the Company’s plans for the three months ended March 31, 2019 is as follows:
 
Number of
Shares
Weighted Average Exercise Price
Options outstanding at December 31, 2018 6,839,417  $ 7.63 
Options granted 688,084  $ 19.12 
Less:
Options exercised 635,257  $ 6.23 
Options canceled or expired 2,500  $ 8.04 
Options outstanding at March 31, 2019 6,889,744  $ 8.84 
Exercisable at Exercisable at March 31, 2019 2,584,373  $ 6.84 
The fair value of each stock option award granted during the three months ended March 31, 2019 was estimated as of the grant date using a trinomial lattice model with the following weighted average assumptions:
  Three Months Ended
March 31, 2019
Expected term (in years) 3.0 - 4.5
Risk-free interest rate (%) 2.5%   
Expected volatility (%) 38.9% - 44.0%
Dividend yield (%) —   
Weighted average fair value/share at grant date $5.64 
 
As of March 31, 2019, there was approximately $7.5 million of unrecognized share based compensation expense related to stock options that will be recognized over a weighted-average period of approximately 1.4 years.  

A summary of the restricted stock activity under the Company’s plans for the three months ended March 31, 2019 is as follows:
 
Number of Restricted
Shares
Weighted Average Grant Date Fair Value
Nonvested at December 31, 2018 282,508  $ 9.01 
Granted 182,502  $ 19.20 
Vested —  $ — 
Nonvested at March 31, 2019 465,010  $ 13.01 

Employee Stock Purchase Plan (ESPP)

We offer an ESPP through which eligible employees may purchase shares of our common stock at a discount of 15% of the fair market value of the Company’s common stock. During the three months ended March 31, 2019 and 2018, employees purchased 36,154 and 36,922 shares, respectively under the ESPP.  The expense recorded for each of periods was approximately $0.1 million.