Under Indiana's true tax value system, improvements are assigned various grades based on their materials, design, and workmanship; the grades represent multipliers that are applied to the base reproduction cost of an improvement. IND. ADMIN. CODE tit. 50, r. 2.2-10-3; Whitley Prods., 704 N.E.2d at 1116. The selection of which grade should be applied to an improvement calls for a subjective judgment and is committed to the discretion of the assessor. Mahan v. State Bd. of Tax Comm'rs, 622 N.E.2d 1058, 1064 (Ind. Tax Ct. 1993). Thus, in determining grade, the assessor is to "distinguish significant variations in an improvement's quality and design." IND. ADMIN. CODE tit. 50, r. 2.2-10-3(a). The State Board's regulations define the different characteristics that help assessors differentiate between grades. for instance, "'D' grade buildings are constructed with economy materials and fair workmanship. These buildings are devoid of architectural treatment and have a substandard quality interior finish." IND. ADMIN. CODE tit. 50, r. 2.2-10-3(a)(4). On the other hand, "'E' grade buildings are constructed with substandard grade materials, usually 'culls' and 'seconds', and very poor quality workmanship resulting from unskilled, inexperienced, do-it-yourself labor." IND. ADMIN. CODE tit. 50, r. 2.2-10-3(a)(5). When contesting a grade assigned to an improvement, a taxpayer must offer probative evidence concerning the alleged assessment error. Whitley Prods., 704 N.E.2d at 1119. A taxpayer's conclusory statements concerning the grading of a subject improvement, however, do not constitute probative evidence. Id., 704 N.E.2d 1113 Likewise, mere references to photographs or State Board regulations, without explanation, do not qualify as probative evidence for purposes of grading issues. Heart City Chrysler v. State Board of Tax Comm'rs, 714 N.E.2d 329, 333 (Ind. Tax Ct. 1999). In the event that a taxpayer fails to provide the State Board with probative evidence supporting its position on a grade issue, the State Board's duty to support its final determination with substantial evidence is not triggered. Whitley Prods., 704 N.E.2d at 1120.