Table:
|  | 2019 | 2018 | 2017 |
| --- | --- | --- | --- |
|  |  | (in thousands) |  |
| Restricted Stock Units: |  |  |  |
| Non-vested at January 1 | 2,454 | 3,174 | 3,778 |
| Awarded | 1,139 | 790 | 939 |
| Vested | (1,237) | (1,382) | (1,434) |
| Forfeited | (87) | (128) | (109) |
| Non-vested at December31 | 2,269 | 2,454 | 3,174 |
| Stock Options: |  |  |  |
| Outstanding at January 1 | 506 | 531 | 926 |
| Granted | 102 | 69 | 111 |
| Exercised | (280) | (94) | (501) |
| Forfeited | (7) | — | — |
| Expired | (2) | — | (5) |
| Outstanding at December 31 | 319 | 506 | 531 |
| Vested and expected to vest at December 31 | 319 | 506 | 531 |
| Exercisable at December 31 | 85 | 256 | 233 |
Paragraphs:
Teradyne determined the stock options’ expected life based upon historical exercise data for executive officers, the age of the executive officers and the terms of the stock option grant. Volatility was determined using historical volatility for a period equal to the expected life. The risk-free interest rate was determined using the U.S. Treasury yield curve in effect at the time of grant. Dividend yield was based upon an estimated annual dividend amount of $0.36 per share divided by Teradyne’s stock price on the grant date of $37.95 for the 2019 grants, $47.70 for the 2018 grants and $28.56 for the 2017 grants.
Stock compensation plan activity for the years 2019, 2018, and 2017, is as follows:
Question:
How was volatility determined?
Answer: using historical volatility for a period equal to the expected life