Illinois Appellate Court - Department improperly used math error procedure to change taxpayer's apportionment factor

An Illinois appellate court found that the Department of Revenue improperly changed a taxpayer's apportionment factor pursuant to a "mathematical error" correction. The court held that the math error correction procedure was improperly utilized on a substantive matter regarding the application of an apportionment statute to a short period return. This decision highlights the procedural differences between a notice of deficiency and a notice of mathematical error in Illinois.