pre_text: The provision for income taxes differed from the provision computed by applying the Federal statutory rate to income (loss) from continuing operations before taxes due to the following: The effective income tax rate was 18.9% and (141.8)% during the years ended December 31, 2019 and December 31, 2018, respectively. The decrease in 2019 compared to statutory tax rate of 21% was primarily due to deferred tax adjustments related to foreign tax credit carryforwards and state taxes, offset by changes in the valuation allowance and excess tax benefits resulting from the exercise of non-qualified stock options. The effective tax rate for the year ended December 31,2018 was significantly impacted by recording a substantial increase in a valuation allowance on the entire deferred tax assets.
table: Year ended December 31,
table: 2019 2018
table: Federal statutory tax rate 21.0% 21.0%
table: State taxes (4.5) 4.4
table: Non deductible expenses (0.3) (0.6)
table: Tax credits 4.0 4.6
table: Expired tax credit (1.3) (3.9)
table: Deferred tax adjustment (4.8) —
table: Stock based compensation 1.9 0.8
table: Valuation allowance 3.2 (167.0)
table: Contingent purchase revaluation — (1.0)
table: Other (0.3) (0.1)
table: 18.9% (141.8)%
question: Analyse this data from a financial earnings document. What is the change in Tax credits from December 31, 2018 and 2019?