related to our acquisition of Pulse. As a percentage of revenue, cost of sales decreased from 60% to 57%, primarily due to an increase in sales volume, a favorable product mix and an increase in the proportion of product revenue to total revenue, partially offset by an increase in reserves for excess and obsolescence inventory.
Gross Margin. Gross margin for the nine months ended January 25, 2020 was $99.9 million, as compared to $91.4 million for the nine months ended January 26, 2019, representing an increase of $8.6 million, or 9%. The increase in gross margin was primarily due to an increase in product margin of $8.2 million and an increase in service margin of $0.4 million. The increase in product margin was primarily due to an increase in product deliveries and a favorable product mix, partially offset by an increase in reserves for excess and obsolescence inventory. Product gross margin for the nine months ended January 25, 2020 included $1.8 million of intangible asset amortization related to our acquisition of Pulse. The increase in service margin was primarily due to a favorable mix. As a percentage of revenue, gross margin increased from 40% to 43%, primarily due to a favorable mix and an increase in the proportion of product revenue to total revenue, partially offset by an increase in reserves for excess and obsolescence inventory.
Selling, General and Administrative. SG&A expense for the nine months ended January 25, 2020 was $43.1 million, or 19% of revenue, compared to SG&A expense of $40.1 million, or 18% of revenue, for the nine months ended January 26, 2019. The increase in SG&A expense was primarily due to an increase in commission on international sales, litigation and employee related expenses, partially offset by a decrease in corporate development expenses primarily related to the sale of our EES business.
Research and Development. R&D expense for the nine months ended January 25, 2020 was $30.9 million, or 13% of revenue, compared to R&D expense of $22.6 million, or 10% of revenue, for the nine months ended January 26, 2019. R&D expense increased by $8.3 million, or 37%, for the nine months ended January 26, 2019, primarily due to an increase in development activities for certain strategic initiatives.
Interest Income, net. Interest income, net for the nine months ended January 25, 2020 was $3.7 million compared to interest income, net of $3.2 million for the nine months ended January 26, 2019. The increase in interest income was primarily due to an increase in the average interest rates earned on our investment portfolio.
Other Income, net. Other income, net, for the nine months ended January 25, 2020 was $0.6 million compared to other income, net of $10.6 million for the nine months ended January 26, 2019. The decrease in other income, net was primarily due to a litigation settlement and income earned under a transition services agreement with the buyer of our former EES Business during the nine months ended January 26, 2019.
Provision for Income Taxes. Our effective income tax rate was 10.6% for the nine months ended January 25, 2020, as compared to 11.1% for the nine months ended January 26, 2019. The decrease in effective income tax rate was primarily due to an increase in estimated federal R&D credits.
Equity Method Investment Loss, net of tax. Equity method investment loss, net of tax for the nine months ended January 25, 2020 was $3.4 million compared to equity method investment loss, net of tax of $2.1 million for the nine months ended January 26, 2019. The increase was due to the equity method loss associated with our investment in the HAPSMobile joint venture formed in December 2017.
Backlog
Consistent with ASC 606, we define funded backlog as remaining performance obligations under firm orders for which funding is currently appropriated to us under a customer contract. As of January 25, 2020, our funded backlog was approximately $126.0 million.
In addition to our funded backlog, we also had unfunded backlog of $41.5 million as of January 25, 2020. Unfunded backlog does not meet the definition of a performance obligation under ASC Topic 606. We define unfunded backlog as the total remaining potential order amounts under cost reimbursable and fixed price contracts with multiple one-year options and indefinite delivery, indefinite quantity (“IDIQ”) contracts. Unfunded backlog does not obligate the U.S. government to purchase goods or services. There can be no assurance that unfunded backlog will result in any orders in