(a) The company defines free cash flow as cash provided by operating activities (a measure prescribed by generally accepted accounting principles) less capital expenditures for property, plant and equipment. The company believes that this supplemental non-GAAP information is useful to assist management and the investment community in analyzing the company’s ability to generate cash flow.

Income Taxes

The effective tax rate for the fourth quarter of 2020 was 6.4% compared with 10.7%. The fourth quarter of 2020 reflected net discrete income tax benefits of $18.8 million, which included a $5.7 million income tax benefit related to share-based accounting. The fourth quarter of 2020 also included net discrete income tax benefits of $9.8 million primarily related to U.S. export sales, U.S. research credits and other items. The fourth quarter of 2019 reflected net discrete income tax benefits of $8.3 million. This amount included $4.2 million in income tax benefit related to share-based accounting and a $4.2 million in income tax benefit primarily related to U.S. export sales. Excluding the net discrete income tax benefits in both periods, the effective tax rates would have been 19.7% for the fourth quarter of 2020 compared with 17.1%.

Other

Stock option expense was $5.9 million for the fourth quarter of 2020 compared with $5.7 million. Stock option expense for fiscal year 2020 was $24.7 million, compared with $26.1 million for fiscal year 2019. Non-service retirement benefit income was $3.2 million for the fourth quarter of 2020, compared with $1.9 million. Interest expense, net of interest income, decreased to $3.4 million for the fourth quarter of 2020 compared with $4.7 million and reflected the impact of lower average interest rates. Corporate expense decreased to $14.7 million for the fourth quarter of 2020, compared with $16.1 million and reflected lower compensation and travel expense.