3. SUPPLEMENTAL FINANCIAL STATEMENT DATA

Accounts Receivable

Accounts receivable consisted of the following:


	December 27,		December 28,
	2019		2018
	(in thousands)
Billed	$	58,651	$	61,788
Unbilled (1)		96,041		45,158
Contract retentions		5,377		6,693
Other assets (2)		14,075		1,626
		174,144		115,265
Allowance for doubtful accounts		(1,147)		(442)
	$	172,997	$	114,823

⁽¹⁾

Unbilled portion represents contract assets which is presented separately from accounts receivable on the consolidated balance sheets.

⁽²⁾

Other assets represents a portion of receivables greater than one year from the normal course of business presented separately from current assets on the consolidated balance sheets.

The movements in the allowance for doubtful accounts consisted of the following:


	Fiscal Year
	2019		2018		2017
	(in thousands)
Balance as of the beginning of the year	$	442	$	369	$	785
(Recovery of) provision for doubtful accounts		1,051		470		(189)
Write-offs of uncollectible accounts		(346)		(397)		(227)
Balance as of the end of the year	$	1,147	$	442	$	369

Billed accounts receivable represent amounts billed to clients that have yet to be collected. Unbilled accounts receivable represent revenue recognized, but not yet billed, pursuant to contract terms or accounts billed after the period end. Substantially all unbilled receivables as of December 27, 2019 and December 28, 2018 are, or were expected to be, billed and collected within twelve months of such date. Contract retentions represent amounts invoiced to clients where payments have been withheld pending the completion of certain milestones, other contractual conditions or upon the completion of the project. These retention agreements vary from project to project and could be outstanding for several months.

Allowances for doubtful accounts have been determined through specific identification of amounts considered to be uncollectible and potential write‑offs, plus a non‑specific allowance for other amounts for which some potential loss has been determined to be probable based on current and past experience.

The Los Angeles Department of Water and Power (“LADWP”) accounted for 16% of the Company’s outstanding receivables as of December 27, 2019. The Dormitory Authority-State of New York (“DASNY”) accounted for 21% of the Company’s outstanding receivables as of December 28, 2018.

From time to time, in connection with factoring agreements, the Company sells trade accounts receivable without recourse to third party purchasers in exchange for cash. During 2019, the Company sold trade accounts receivable and received cash proceeds of $4.9 million. The discounts on the trade accounts receivable sold during 2019 were $0.2 million and were recorded within “Other” general and administrative expenses in the consolidated financial statements. During 2018, the Company did not sell any trade accounts receivable.

Equipment and Leasehold Improvements

Equipment and leasehold improvements were as follows:


	December 27,		December 28,
	2019		2018
	(in thousands)
Furniture and fixtures	$	4,614	$	3,551
Computer hardware and software		14,789		10,874
Leasehold improvements		2,410		1,419
Equipment under finance leases		1,957		1,304
Automobiles, trucks, and field equipment		3,564		2,635
		27,334		19,783
Accumulated depreciation and amortization		(15,283)		(11,785)
Equipment and leasehold improvements, net	$	12,051	$	7,998

Depreciation expense of equipment and leasehold improvements totaled $3.4 million, $1.6 million, and $1.5 million in fiscal years 2019, 2018, and 2017, respectively.

Included in accumulated depreciation and amortization is $509,000, $374,000 and $380,000 of amortization expense related to equipment held under finance leases in fiscal years 2019, 2018, and 2017, respectively.

Accrued Liabilities

Accrued liabilities were as follows:


	December 27,		December 28,
	2019		2018
	(in thousands)
Accrued subcontractor costs	$	45,366	$	21,446
Accrued bonuses		7,756		5,273
Other		4,630		2,489
Compensation and payroll taxes		3,286		2,544
Employee withholdings		3,463		2,137
Paid leave bank		3,114		3,512
Total accrued liabilities	$	67,615	$	37,401