Selling, General and Administrative. Selling, general and administrative expense, which is primarily comprised of compensation expense, increased by $1.6 million, or 1.3%, to $123.5 million in the three months ended December 31, 2019, from $121.9 million in the three months ended December 31, 2018. Selling, general and administrative expense decreased as a percentage of revenue from 12.0% in the three months ended December 31, 2018, to 11.8% in the three months ended December 31, 2019. Selling, general and administrative expense may fluctuate from time to time, depending upon such factors as changes in our workforce and sales efforts and the results of any operational efficiency programs that we may undertake.
Amortization of Purchased Intangible Assets and Other.Amortization of purchased intangible assets and other in the three months ended December 31, 2019, decreased by $4.6 million, or 17.6% to $21.3 million from $25.8 million in the three months ended December 31, 2018. The decrease in amortization of purchased intangible assets and other was primarily attributable to a completion of amortization of previously purchased intangible assets, partially offset by an increase in amortization of intangible assets due to acquisitions completed in fiscal year 2019.
Operating Income. Operating income increased by $10.5 million, or 7.9%, in the three months ended December 31, 2019, to $143.6 million, or 13.8% of revenue, from $133.1 million, or 13.2% of revenue, in the three months ended December 31, 2018. The increase in operating income was attributable primarily to the increase in revenue partially offset by the increase in cost of revenue, and to the decrease in amortization of purchased intangible assets and other during the three months ended December 31, 2019. Negative foreign exchange impacts on our revenue were partially offset by the negative foreign exchange impacts on our operating expense, resulting in an immaterial negative impact on our operating income.
Interest and Other (Expense) Income, Net.Interest and other (expense) income, net, changed from a net gain of $1.5 million in the three months ended December 31, 2018 to a net loss of $0.4 million in the three months ended December 31, 2019. The change in interest and other (expense) income, net, was primarily attributable to the realized gain of a minority equity investment recorded in the three months ended December 31, 2018, offset by decrease in foreign exchange impacts.
Income Taxes. Income taxes for the three months ended December 31, 2019 were $27.3 million onpre-tax income of $143.2 million, resulting in an effective tax rate of 19.1%, compared to 24.5% in the three months ended December 31, 2018. Our effective tax rate may fluctuate between periods as a result of discrete items that may affect a particular period. Please see Note 9 to our consolidated financial statements.
Net Income. Net income increased by $14.2 million, or 14.0%, to $115.9 million in the three months ended December 31, 2019, from $101.7 million in the three months ended December 31, 2018. The increase in net income was attributable to the increase in Operating Income coupled with decrease in income taxes.
Diluted Earnings Per Share.Diluted earnings per share increased by $0.13, or 18.1%, to $0.85 in the three months ended December 31, 2019, from $0.72 in the three months ended December 31, 2018. The increase in diluted earnings per share was primarily attributable to the increase in net income as well as to the decrease in the diluted weighted average number of shares outstanding which resulted from share repurchases. Please see also Note 10 to our consolidated financial statements.
Liquidity and Capital Resources
Cash, Cash Equivalents and Short-Term Interest-Bearing Investments. Cash, cash equivalents and short-term interest-bearing investments, totaled $485.9 million as of December 31, 2019, compared to $471.6 million as of September 30, 2019. The increase was mainly attributable to $163.9 million in positive cash flow from operations and $41.2 million of proceeds from stock option exercises, partially offset $90.0 million repurchase of our ordinary shares, $58.5 million for capital expenditures, net, $38.4 million of cash dividend payment and $1.4 million contingent consideration payment from a business acquisition. Net cash provided by operating activities amounted to $163.9 million and $109.7 in the three months ended December 31, 2019 and 2018, respectively.
Our free cash flow for the three months ended December 31, 2019 was $105.4 million and is calculated as net cash provided by operating activities of $163.9 million for the period less $58.5 million for capital expenditures, net.
Free cash flow is anon-GAAP financial measure and is not prepared in accordance with, and is not an alternative for, generally accepted accounting principles and may be different fromnon-GAAP financial measures with similar names used by other companies.Non-GAAP measures such as free cash flow should only be reviewed in conjunction with the corresponding GAAP measures. We believe that free cash flow when used in conjunction with the corresponding GAAP measure provides useful information to investors and management relating to the amount of cash generated by the Company’s business operations.
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