by an increase of $2.5 million in intangible asset amortization expense associated with our acquisition of Pulse Aerospace in June 2019. The increase in services margins was primarily due to the increase in services revenue and a favorable service mix. As a percentage of revenue, gross margin increased from 41% to 42%, primarily due to an increase in the proportion of product sales to total revenue and a favorable service mix, partially offset by acquired intangible asset amortization expense. As a percentage of revenue, product gross margin for fiscal 2020 decreased by nearly 70 basis points to 46%. We anticipate product margin in fiscal year 2021 to continue to decline primarily due to an unfavorable product mix.
Selling, General and Administrative. SG&A expense for the fiscal year ended April 30, 2020 was $59.5 million, or 16% of revenue, compared to SG&A expense of $60.3 million, or 19% of revenue, for the fiscal year ended April 30, 2019. The decrease in SG&A expense was primarily due to a $4.4 million impairment charge related to the long-lived assets of our commercial Quantix product during the fiscal year ended April 30, 2019, a decrease in corporate development expenses primarily related to the sale of our EES Business and a decrease in costs incurred related to the transition services agreement with Webasto, partially offset by an increase in employee-related expenses and an increase in commission expenses associated with an increase in the number of international small UAS contracts under which we utilized sales agents.
Research and Development. R&D expense for the fiscal year ended April 30, 2020 was $46.5 million, or 13% of revenue, compared to R&D expense of $34.2 million, or 11% of revenue, for the fiscal year ended April 30, 2019. R&D expense increased primarily due to increased development activities for certain strategic initiatives.
Interest Income, net. Interest income, net for the fiscal year ended April 30, 2020 was $4.8 million, compared to $4.7 million for the fiscal year ended April 30, 2019. The increase in interest income was primarily due to an increase in the average interest rates earned on our investments portfolio, partially offset by a decrease in our investments balances. Due to the significant decline in market interest rates combined with a shift in our investment composition towards U.S. government and U.S. government agency securities during the fourth quarter of fiscal year 2020, we anticipate interest income earned on our investments portfolio to decrease in future periods.
Other Income (Expense), net. Other income, net for the fiscal year ended April 30, 2020 was $0.7 million, as compared to other income, net of $12.0 million for the fiscal year ended April 30, 2019. The decrease in other income, net was primarily due to a one-time litigation settlement during the fiscal year ended April 30, 2019 and a decrease in income earned under a transition services agreement with Webasto, the buyer of our former EES Business.
Income Taxes. Our effective income tax rate was 11.1% for the fiscal year ended April 30, 2020, as compared to 9.2% for the fiscal year ended April 30, 2019. The increase in our effective tax rate was primarily due to decrease in excess tax benefits from the vesting of employee equity awards and a lower proportion of R&D expense which qualifies for R&D tax credits.
Equity method investment loss, net of tax. Equity method investment loss, net of tax for the fiscal year ended April 30, 2020 was $5.5 million, as compared to equity method investment loss, net of $3.9 million for the fiscal year ended April 30, 2019. The increase was primarily due to the equity method loss associated with our investment in the HAPSMobile joint venture formed in December 2017.
(Loss) gain on sale of business, net of tax. Loss on sale of business, net of tax for the fiscal year ended April 30, 2020 was $0.3 million, as compared to gain on sale of business, net of tax of $8.5 million for the fiscal year ended April 30, 2019. The gain on sale of business, net of tax for the prior year period resulted from the sale of our former EES Business during the fiscal year ended April 30, 2019. We recorded an adjustment related to a settled working capital dispute during the fiscal year ended April 30, 2020.
Loss from discontinued operations, net of tax. Loss from discontinued operations, net of tax for the fiscal year ended April 30, 2020 was $0, as compared to $3.0 million for the fiscal year ended April 30, 2019. The loss from discontinued operations, net of tax for the prior year period related to the results of our EES Business prior to the sale.