This inquiry can be difficult when the challenged agency action is not a formally announced regulation, as in Abbott Labs or Toilet Goods, but an informal pronouncement such as an advice letter.

• 当受到质疑的机关行为并非正式公布的管理行为（如在"艾博特试验室"案和"化妆用品"案里，机关行为只是一种非正式的宣布，如建议性信函）时，上述调查会难度很大。