We establish the amount of our allowance for doubtful accounts based upon factors relating to the credit risk of specific customers, current market conditions, and other information. Our allowance for doubtful accounts was approximately $6.3 million and $6.1 million at June 30, 2019, and December 31, 2018, respectively.

Revenues related to flights generated by our Air Medical segment are recorded net of contractual allowances under agreements with third party payors and estimated uncompensated care when the services are provided. The allowance for contractual discounts was $140.5 million and $123.8 million as of June 30, 2019 and December 31, 2018, respectively. The allowance for uncompensated care was $45.5 million and $51.9 million as of June 30, 2019 and December 31, 2018, respectively.

Included in the allowance for uncompensated care listed above is the value of services to patients who are unable to pay when it is determined that they qualify for charity care. The value of these services was $1.6 million and $1.7 million for the quarters ended June 30, 2019 and 2018, respectively. The value of these services was $3.5 million and $3.3 million for the six months ended June 30, 2019 and 2018, respectively. The estimated cost of providing charity services was $0.3 million and $0.4 million for the quarters ended June 30, 2019 and 2018, respectively. The estimated cost of providing charity services was $0.8 million for the six months ended June 30, 2019 and 2018, respectively. The estimated costs of providing charity services are based on a calculation that applies a ratio of costs to the charges for uncompensated charity care. The ratio of costs to charges is based on our Air Medical segment’s total expenses divided by gross patient service revenue.

The allowance for contractual discounts and estimated uncompensated care (expressed as a percentage of gross segment accounts receivable) as of the dates listed below was as follows:

	June 30, 2019	December 31, 2018
Allowance for Contractual Discounts	55%	51%
Allowance for Uncompensated Care	18%	22%

We have also established valuation reserves related to obsolete and slow-moving spare parts inventory. The inventory valuation reserves were $19.4 million and $17.9 million at June 30, 2019 and December 31, 2018, respectively.