Inventories

Inventories are valued at the lower of cost or net realizable value. Cost is determined by the first-in, first-out (“FIFO”) method. We establish an allowance for excess and obsolete inventories based on product line changes, the feasibility of substituting parts and the need for supply and replacement parts.

The components of inventories are as follows:



	March 31, 2019			December 31, 2018
	(in thousands)
Raw materials	$	67,519		$	67,995
Work in process	3,131			4,060
Finished goods	5,494			6,767
	76,144			78,822
Less: Allowance for excess and obsolete inventories	(1,567		)	(1,210		)
Total, net	$	74,577		$	77,612

The related changes in the allowance for excess and obsolete inventories account are as follows:



	Three months ended
	March 31, 2019		March 31, 2018
Allowance for excess and obsolete inventories:	(in thousands)
Balance, beginning of period	$	1,210	$	1,118
Provisions for excess and obsolete inventories	357		101
Inventories written off	—		(10		)
Balance, end of period	$	1,567	$	1,209