a percentage of as adjusted non-GAAP net revenue increased by 1.2% to 5.5% for the six months ended June 30, 2019, from 4.3% for the six months ended June 30, 2018 due to the increase in professional fees, employee compensation, and office space, partially offset by our ability to leverage our general and administrative expenses across an increasing revenue base.
Amortization of intangible assets. The amortization of intangible assets for the six months ended June 30, 2019 was $3.9 million and is related to the Acquisition of Orbis Education, which resulted in the creation of certain identifiable intangible assets that will be amortized over their expected lives.
Loss on transaction. The loss on transaction for the six months ended June 30, 2019 was $4.0 million due to transaction costs related to the Acquisition of Orbis Education, an increase of $2.0 million, as compared to the loss of $2.0 million related to the GCU Transaction for the six months ended June 30, 2018.
University related expenses. Our university related expenses for the six months ended June 30, 2018 were $167.2 million. The expenses included in the six months ended June 30, 2018 represent university related expenses for activities that have now transferred to GCU and are not related to our current business activities as a service provider.
Interest income on Secured Note. Interest income on the Secured Note (as defined below) for the six months ended June 30, 2019 was $28.2 million. As a result of the Transaction with GCU on July 1, 2018, the Company recognizes interest income on its Secured Note with GCU including borrowings made for capital expenditures, earning interest at 6%, with monthly interest payments.
Interest expense. Interest expense was $5.5 million for the six months ended June 30, 2019, and increase of $5.1 million, as compared to interest expense of $0.4 million for the six months ended June 30, 2018. The increase in interest expense is primarily due to the Acquisition of Orbis Education, which resulted in a $190.1 million increase in our outstanding credit facility, a slightly higher interest rate on the credit facility, additional fees on a new revolving credit facility, and lower capitalized interest as compared to the same period in the prior year.
Investment interest and other. Investment interest and other for the six months ended June 30, 2019 was $3.8 million, an increase of $1.3 million, as compared to $2.5 million in the six months ended June 30, 2018.
Income tax expense. Income tax expense for the six months ended June 30, 2019 was $25.6 million, a decrease of $5.4 million, or 17.5%, as compared to income tax expense of $31.0 million for the six months ended June 30, 2018. This decrease is the result of a decrease in our effective tax rate and a slight decrease in our taxable income between periods. Our effective tax rate was 17.1% during the six months ended June 30, 2019 compared to 20.6% during the six months ended June 30, 2018. The decrease in the effective tax rate resulted from an agreement with the Arizona Department of Revenue regarding previously filed refund claims related to income tax obligations for prior calendar years, which resulted in a favorable tax impact of $5.9 million recorded as a discrete tax item in the first quarter of 2019. In addition, in the second quarter of 2019, the effective tax rate was favorably impacted by a recent law change with respect to Arizona state taxes, and an increase in excess tax benefits to $6.8 million from $6.5 million in the six months ended June 30, 2019 and 2018, respectively. The inclusion of excess tax benefits and deficiencies as a component of our income tax expense will increase volatility within our provision for income taxes as the amount of excess tax benefits or deficiencies from share-based compensation awards are dependent on our stock price at the date the restricted awards vest, our stock price on the date an option is exercised, and the quantity of options exercised. Our restricted stock vests in March each year so the favorable benefit will primarily impact the first quarter each year.
Net income. Our net income for the six months ended June 30, 2019 was $124.4 million, an increase of $4.7 million, as compared to $119.7 million for the six months ended June 30, 2018, due to the factors discussed above.
Seasonality
Our net revenue and operating results normally fluctuate as a result of seasonal variations in our business, principally due to changes in our university partners’ enrollment. Our partners’ enrollment varies as a result of new enrollments, graduations, and student attrition. Revenues in the summer months (May through August) are lower