Calculate the return on common stockholders' equity ratio for 2018 and 2017. Stockholders' equity at December 31, 2016, was $897,431 (in thousands). Why is the denominator an average instead of a single amount?

Consolidated Statement of Earnings and Retained Earnings
Year Ended December 31,	2018	2017	2016
Earnings	(in thousands)
Revenues:
Net sales	$2,004,719	$1,937,021	$1,835,987
Investment and other income	18,636	17,153	14,614
Total revenues	$2,023,355	$1,954,174	$1,850,601
Costs and expenses:
Cost of goods sold	$ 848,363	$ 847,366	$ 814,483
Costs (gain) related to factory closure and sale	-10,404	3,300	10,436
Selling, distribution, and general administrative	743,902	708,310	656,473
Interest	615	958	1,097
Total cost and expenses	$1,582,476	$1,559,934	$1,482,489
Earnings before income taxes	$ 440,879	$ 394,240	$ 368,112
Income taxes	136,378	122,614	128,840
Net earnings	$ 304,501	$ 271,626	$ 239,272
Retained Earnings
Retained earnings at beginning of year	$1,032,139	$ 898,512	$ 497,481
Dividends declared
(per share: 2018$1.31; 2017$1.19)	(152,023)	(137,999)	(87,301)
Retained earnings at end of year	$1,184,617	$1,032,139	$ 649,452
Per-Share Amounts
Net earnings per average share of common stock	$2.63	$2.34	$1.99
Dividends paid per share of common stock	$1.30	$1.17	$1.02
Culinary Delights Company
Consolidated Statement of Earnings and Retained Earnings
Year Ended December 31,	2018	2017
ASSETS	(in thousands)
Current assets:
Cash and cash equivalents	$ 214,572	$ 206,627
Short-term investments, at amortized cost	137,112	120,728
Accounts receivable, net	194,877	175,967
Inventory	256,108	247,392
Other prepaid assets	25,376	30,538
Deferred income taxescurrent	15,027	16,421
Total current assets	$ 843,072	$ 797,673
Marketable equity securities, at fair value	39,888	26,375
Deferred charges and other assets	92,183	59,566
Deferred income taxesnoncurrent	25,522	29,038
Property, plant, and equipment, at cost:
Land	$ 36,013	$ 26,298
Buildings and building equipment	310,212	277,808
Machinery and equipment	642,556	566,766
	$ 988,781	$ 870,872
Less accumulated depreciation	468,691	440,398
	$ 520,090	$ 430,474
Total assets	$1,520,755	$1,343,126
LIABILITIES AND STOCKHOLDERS EQUITY	(in thousands)
	2018	2017
Current liabilities:
Accounts payable	$ 76,691	$ 71,001
Accrued expenses	67,848	78,378
Dividends payable	23,222	22,034
Income and other taxes payable	49,491	53,460
Deferred income taxescurrent	1,374	943
Total current liabilities	$ 218,626	$ 225,816
Deferred income taxesnoncurrent	$ 40,312	$ 30,874
Other noncurrent liabilities	104,885	101,057
Stockholders' equity:
Preferred stockno par, authorized 20,000 shares; issued 0 shares
Common stockno par, authorized 400,000 shares; issued 2018: 93,007; 2017: 92,545	$ 12,401	$ 2,339
Class B commonconvertible authorized 80,000 shares; issued 2018: 23,214; 2017: 23,676	3,095	3,157
Additional paid-in capital	272	226
Retained earnings	1,184,617	1,032,139
Foreign currency translation adjustment	-61,339	-65,034
Unrealized holding gains on marketable equity securities	24,698	25,915
Common stock in treasury, at cost (2018: 111 shares; 2017: 252 shares)	(6,712)	(13,363)
Total stockholders' equity	$1,157,032	$ 985,379
Total liabilities and stockholders' equity	$1,520,855	$1,343,126