Maintenance cost at a hospital was observed to increase as activity level increased. The following data was gathered:

Activity Level -

Maintenance Cost

Month

Patient Days

Incurred

January

5,600

$7,900

February

7,100

$8,500

March

5,000

$7,400

April

6,500

$8,200

May

7,300

$9,100

June

8,000

$9,800

If the cost of maintenance is expressed in an equation, what is the independent variable for this data?