The function of the board of equalization shall be confined strictly to assuring that the market value for assessment purposes of property has been found by the assessor, and to the functions provided for in chapter 6, title 63, Idaho Code, relating to exemptions from taxation. It is hereby made the duty of the board of equalization to enforce and compel a proper classification and assessment of all property required under the provisions of this title to be entered on the property rolls, and in so doing, the board of equalization shall examine the rolls and shall raise or cause to be raised, or lower or cause to be lowered, the assessment of any property which in the judgment of the board has not been properly assessed. The board of equalization must examine and act upon all complaints filed with the board in regard to the assessed value of any property entered on the property rolls and must correct any assessment improperly made. The taxpayer shall have the burden of proof in seeking affirmative relief to establish that the determination of the assessor is erroneous, including any determination of assessed value. A preponderance of the evidence shall suffice to sustain the burden of proof.