product mix, as well as nine months of costs incurred from the amortization of the Uman acquisition-related inventory valuation adjustment and acquired intangibles during the nine months ended September 30, 2020, as compared to only three months of these costs during the nine months ended September 30, 2019. Cost of service revenue increased to $8.1 million for the nine months ended September 30, 2020 from $6.6 million for the nine months ended September 30, 2019. The increase was primarily due to higher utilization of the Accelerator Laboratory, plus increased personnel costs from the build out of our field service organization. Cost of collaboration and license revenue of $1.0 million resulted from the licensing of certain technology and intellectual property to Abbott during the nine months ended September 30, 2020. We had no cost of collaboration and license revenue during the nine months ended September 30, 2019. Overall cost of goods sold as a percentage of revenue decreased to 45% of total revenue for the nine months ended September 30, 2020 as compared to 51% for the nine months ended September 30, 2019, primarily as a result of the significant increase in collaboration and license revenue.
Research and Development Expense
Research and development expense increased by $2.2 million, or 18%, to $14.0 million for the nine months ended September 30, 2020 as compared to $11.8 million for the nine months ended September 30, 2019. The increase was primarily due to compensation, development, materials, and other expenses related to work under the RADx WP1 award incurred during the nine months ended September 30, 2020.
Selling, General, and Administrative Expense
Selling, general, and administrative expense increased by $2.5 million, or 7%, to $40.8 million for the nine months ended September 30, 2020 as compared to $38.3 million for the nine months ended September 30, 2019. The increase was primarily due to headcount additions in various departments as we build out our organization to support future growth, the lease for the new headquarters, and stock compensation expense.
Interest Income (Expense) and Other Expense, Net
Interest income (expense) and other expense, net decreased by $0.5 million for the nine months ended September 30, 2020 as compared to the same period in 2019, primarily due to the unfavorable impact of COVID-19 on the interest rates of our cash equivalents during the nine months ended September 30, 2020.
Income Tax Benefit
Income tax benefit was $0.3 million for the nine months ended September 30, 2020 as compared to $0.1 million for the same period in 2019. The change is primarily due to certain state and international taxes in 2020.
Liquidity and Capital Resources
To date, we have financed our operations principally through equity offerings, borrowings from credit facilities and revenue from our commercial operations.
Equity Offerings
In December 2017, we completed our initial public offering (IPO) in which we sold 4,916,480 shares of common stock at a price of $15.00 per share. The aggregate net proceeds received by us from the offering, net of underwriting discounts and commissions and offering expenses, were $65.6 million. Prior to the IPO, we had raised capital through the sale of redeemable convertible preferred stock in private placement transactions.
On March 19, 2019, we entered into a Sales Agreement for an “at the market offering” arrangement with Cowen and Company, LLC (Cowen), which allows us to issue and sell shares of common stock pursuant to a shelf registration statement for total gross sales proceeds of up to $50.0 million from time to time through Cowen, acting as our agent. During the 2019 fiscal year, we sold an aggregate of 2,186,163 shares of common stock pursuant to this