Final answer:
Companies must adhere to the consistency concept in accounting to ensure that financial statements are comparable and reliable over different periods.
Step-by-step explanation:
Companies are not allowed to switch back and forth between alternative accounting methods from year to year because of the consistency concept. This fundamental accounting principle ensures that a company's financial statements are comparable across periods and that the financial data is reliable for users, including investors, creditors, and others who may review the financial statements. Constantly changing accounting methods would lead to inconsistencies, making it difficult to compare financial data over time and potentially misleading users of the financial statements.