68 Determining the amount of stock-based compensation expense to be recorded requires the Company to develop estimates used in calculating the grant-date fair value of awards. These estimates may be based on different valuation models depending upon the type of award and may include assumptions, such as expected volatility, expected term, risk-free interest rate, expected dividend yield, forfeiture rate and others. The Company uses the Black-Scholes valuation model to calculate the grant-date fair value of stock option awards. The grant-date fair value of restricted stock units with a service condition and restricted stock units with both service and performance conditions are calculated using the value of the Company's common stock on the date of grant, reduced by the present value of dividends expected to be paid on the Company's common stock prior to vesting. For restricted stock units with both service and performance conditions, this grant-date fair value is also impacted by the number of units that are expected to vest during the performance period and is adjusted through the related stock-based compensation expense at each reporting period based on the probability of achievement of that performance condition. If the Company determines that an award is unlikely to vest, any previously recorded stock-based compensation expense is reversed in the period of that determination. The grant date fair value of restricted stock units with both service and market conditions is calculated using the Monte Carlo simulation model to estimate the probability of satisfying the performance condition stipulated in the award grant, including the possibility that the market condition may not be satisfied. See Note 3,  Stock-Based Compensation and Shareholders' Equity , of the Notes to Consolidated Financial Statements for additional information relating to stock-based compensation. Stock-based compensation is measured at the grant date based on the grant-date fair value of the awards ultimately expected to vest and is recognized as an expense on a straight-line basis over the vesting period, which is generally four years for stock options and restricted stock units, or in annual installments of 25% on each of the first, second, third and fourth anniversaries of the date of grant. Restricted stock units with service and performance or market conditions generally vest over a three-year performance period. For grants issued prior to fiscal 2018, the vesting period was generally five years for stock options, or in annual installments of 20% on each of the first, second, third, fourth and fifth anniversaries of the date of grant and in one installment on the third anniversary of the date of grant for restricted stock units/awards. The maximum contractual term of all stock options is ten years. r. Stock-Based Compensation (1) Balances have been restated to reflect the adoption of ASU 2014-09. See Note 2a,  Principles of Consolidation , of the Notes to Consolidated Financial Statements. (2) For the year ended October 31, 2020, the amount is not material. The following table sets forth the computation of basic and diluted earnings per share: 2020 2019 2018 (1) Net income $ 1,220,761  $ 1,363,011  $ 1,506,980  Less: income allocated to participating securities (2) —  3,229  5,909  Net income allocated to common shareholders $ 1,220,761  $ 1,359,782  $ 1,501,071  Basic shares: Weighted-average shares outstanding 368,633  369,133  370,430  Earnings per common share basic $ 3.31  $ 3.68  $ 4.05  Diluted shares: Weighted-average shares outstanding 368,633  369,133  370,430  Assumed exercise of common stock equivalents 3,340  3,738  4,508  Weighted-average common and common equivalent shares 371,973  372,871  374,938  Earnings per common share diluted $ 3.28  $ 3.65  $ 4.00  Anti-dilutive shares related to: Outstanding stock options 460  826  1,649  ANALOG DEVICES, INC. NOTES TO CONSOLIDATED FINANCIAL STATEMENTS — (Continued)