The Company is exposed to credit losses primarily through the provision of its Executive Search, Consulting and Digital services. The Company’s expected credit loss allowance methodology for accounts receivable is developed using historical collection experience, current and future economic and market conditions and a review of the current status of customers' trade accounts receivables. Due to the short-term nature of such receivables, the estimate of the amount of accounts receivable that may not be collected is primarily based on historical loss-rate experience. When required, the Company adjusts the loss-rate methodology to account for current conditions and reasonable and supportable expectations of future economic and market conditions. The Company generally assesses future economic conditions for a period of sixty to ninety days, which corresponds with the contractual life of its accounts receivables. Additionally, specific allowance amounts are established to record the appropriate provision for customers that have a higher probability of default. The Company’s monitoring activities include timely account reconciliation, dispute resolution, payment confirmation, consideration of customers' financial condition and macroeconomic conditions. Balances are written off when determined to be uncollectible. The Company considered the current and expected future economic and market conditions surrounding COVID-19 as of the end of the fiscal year and determined that the estimate of credit losses was not significantly impacted as of that date.

The activity in the allowance for credit losses on the Company's trade receivables is as follows:

	(in thousands)
Balance at April 30, 2018	$	17,845
Provision for credit losses		14,260
Write-offs		(10,608	)
Recoveries of amounts previously written off		911
Foreign currency translation		(826	)
Balance at April 30, 2019		21,582
Provision for credit losses		14,644
Write-offs		(12,518	)
Recoveries of amounts previously written off		398
Foreign currency translation		(311	)
Balance at April 30, 2020		23,795
Provision for credit losses		15,763
Write-offs		(12,073	)
Recoveries of amounts previously written off		311
Foreign currency translation		1,528
Balance at April 30, 2021	$	29,324

The fair value and unrealized losses on available for sale debt securities, aggregated by investment category and the length of time the security has been in an unrealized loss position, are as follows:

	Less Than 12 Months				Balance Sheet Classification
	Fair Value		Unrealized Loss		Cash and Cash Equivalents		Marketable Securities, Current		Marketable Securities, Non-Current
Balance at April 30, 2021	(in thousands)
Commercial paper	$	36,378	$	7	$	5,749	$	30,629	$	—
Corporate notes/bonds	$	26,350	$	20	$	—	$	10,133	$	16,217

The unrealized losses on 18 investments in Commercial paper securities and 15 investments in Corporate notes/bonds were caused by fluctuations in market interest rates. The Company only purchases high grade bonds that have a maturity from the date of purchase of no more than two years. The Company monitors the credit worthiness of its investments on a quarterly basis. The Company does not intend to sell the investments and does not believe it will be required to sell the investments before the investments mature and therefore recover the amortized cost basis.