The assessor and the assessor's deputies shall take and subscribe the following oath or affirmation:
"I do solemnly swear (or affirm) that I will well and truly discharge the duties of assessor of the county of _________, and will, to the best of my knowledge and ability, truly and fairly determine the valuation, without favor or partiality, of all the taxable property in said county at its full cash value."
A.R.S. § 11-542