Suppose that a surgical ward has gathered the following information for four nursing activities and two types of paitents.

			Patient Category
	Driver	Normal	Intensive	Activity Rate
Treating Patients	Treatments	8000	10,000	$4.00
Providing hygienic care	Hygienic hours	6,000	22,000	$5.00
Responding to requests	Requests	40,000	100,000	$2.00
Monitoring patients	Monitoring hours	7,500	90,000	$3.00

	Normal	Intensive
Treating patients:
(Driver x Activity Rate)	$32,000
(Driver x Activity Rate)		$40,000
Providing hygienic care:
(Driver x Activity Rate)	$30,000
(Driver x Activity Rate)		$110,000
Responding to requests:
(Driver x Activity Rate)	$80,000
(Driver x Activity Rate)		$200,000
Monitoring patients:
(Driver x Activity Rate)	$22,500
(Driver x Activity Rate)		$270,000
Total cost assigned:	$164,500	$620,000
Part 2
Nursing cost per patient per day
	Normal	Intensive
(Total cost assigned / patient days)	$16.45
(Total cost assigned / patient days)		$77.50
Part 3
Total nursing costs Normal (from part 1)	$164,500
Total nursing costs Intensive (from part 1)	$620,000
Total nursing costs	$784,500
Normal patient days	10,000
Intensive patient days	8,000
Total patient days	18,000
Nursing cost per patient day	$43.58

I have determined the total nursing costs assigned to each patient category and I have calculated the nursing cost per day for each type of patient.

My question is: The supervisor of the surgical ward has suggested that patient days is the only driver needed to assign nursing costs to each type of patient. I have calculated the charge per patient day using this approach above. However, why would only using the patient days as the only driver be a bad idea to calculate costs?