Landstar provides for the estimated costs of self-insured claims primarily on an actuarial basis. The amount recorded for the estimated liability for claims incurred is based upon the facts and circumstances known on the applicable balance sheet date. The ultimate resolution of these claims may be for an amount greater or less than the amount estimated by management. The Company continually revises its existing claim estimates as new or revised information becomes available on the status of each claim. Historically, the Company has experienced both favorable and unfavorable development of prior years’ claims estimates.

The following table summarizes the effect of the increase in the cost of insurance claims resulting from unfavorable development of prior year self-insured claims estimates on operating income, net income attributable to Landstar System, Inc. and subsidiary and earnings per share attributable to Landstar System, Inc. and subsidiary set forth in the consolidated statements of income for the thirty-nine-week and thirteen-week periods ended September 28, 2019 and September 29, 2018 (in thousands, except per share amounts):


	Thirty Nine Weeks Ended				Thirteen Weeks Ended
	September 28, 2019		September 29, 2018		September 28, 2019		September 29, 2018
Operating income	$	9,497	$	11,596	$	6,597	$	3,352
Net income attributable to Landstar System, Inc. and subsidiary		7,199		8,755		5,001		2,531
Earnings per share attributable to Landstar System, Inc. and subsidiary	$	0.18	$	0.21	$	0.13	$	0.06
Diluted earnings per share attributable to Landstar System, Inc. and subsidiary	$	0.18	$	0.21	$	0.13	$	0.06

The unfavorable development of prior years’ claims in the thirty-nine-week period ended September 28, 2019 was attributable to several claims as well as actuarially determined adjustments to prior year commercial trucking loss estimates. The unfavorable development of prior years’ claims in the thirty-nine-week period ended September 29, 2018 was primarily attributable to five claims.