Cost of sales. Cost of sales decreased by $2.5 million to $50.4 million in the three months ended March 31, 2021, from $52.9 million in the three months ended March 31, 2020. This decrease was primarily due to a decrease in gallons delivered and a decrease in the cost of station construction activities, partially offset by an increase in our effective cost per gallon.
Our effective cost per gallon increased by $0.02 per gallon to $0.50 per gallon in the three months ended March 31, 2021 from $0.48 per gallon in the three months ended March 31, 2020. Our effective cost per gallon is defined as the total costs associated with delivering our fuels, including commodity costs, transportation fees, liquefaction charges, and other site operating costs, plus the total cost of providing O&M services at stations that we do not own and for which we receive a per-gallon or fixed fee, including direct technician labor, indirect supervisor and management labor, repair parts and other direct maintenance costs, all divided by the total GGEs delivered less GGEs delivered by non-consolidated entities, such as entities that are accounted for under the equity method. The increase in our effective cost per gallon was due to higher commodity prices.
Change in fair value of derivative warrants. Change in fair value of derivative warrants, all of which were issued by our subsidiary, NG Advantage, decreased by $0.4 million to $0.0 million in the three months ended March 31, 2021, from $0.4 million in the three months ended March 31, 2020. The warrants expired unexercised on July 2, 2020.
Selling, general and administrative. Selling, general and administrative expenses increased by $3.1 million to $21.4 million in the three months ended March 31, 2021, from $18.3 million in the three months ended March 31, 2020. This increase was primarily driven by an increase in stock compensation expense due to equity awards granted during the period and the Company’s higher stock price.
Depreciation and amortization. Depreciation and amortization decreased by $0.2 million to $11.7 million in the three months ended March 31, 2021, from $11.9 million in the three months ended March 31, 2020, primarily due to a lower amount of depreciable assets.
Interest expense. Interest expense decreased by $0.8 million to $1.4 million in the three months ended March 31, 2021, from $2.2 million in the three months ended March 31, 2020, primarily due to lower average interest rates on outstanding indebtedness between periods.
Other income, net. Other income, net increased by $0.5 million from $0.2 million in the three months ended March 31, 2020 to $0.7 million in the three months ended March 31, 2021, primarily due to certain other fees earned.
Income (loss) from equity method investments. Income (loss) from equity method investments changed to a loss of $0.4 million for the three months ended March 31, 2021 from income of $0.1 million for the three months ended March 31, 2020, primarily due to the operating results of SAFE&CEC S.r.l.
Income tax expense. Income tax expense was $0.1 million in each of the three months ended March 31, 2021 and 2020, primarily comprised of deferred taxes associated with goodwill and the Company’s expected state tax expense which was consistent between the periods.
Loss attributable to noncontrolling interest. During the three months ended March 31, 2021 and 2020, we recorded a gain of $0.3 million and $0.8 million, respectively, for the noncontrolling interest in the net loss of NG Advantage. The noncontrolling interest in NG Advantage represents a 6.7% minority interest that was held by third parties during both the 2021 and 2020 periods.
Liquidity and Capital Resources
Liquidity
Liquidity is the ability to meet present and future financial obligations through operating cash flows, the sale or maturity of investments or the acquisition of additional funds through capital management. Our financial position and liquidity are, and will continue to be, influenced by a variety of factors, including the level of our outstanding indebtedness