Final answer:
The base amount used in a vertical analysis of an income statement is net sales.
Step-by-step explanation:
In a vertical analysis of an income statement, the base amount is used as a reference point to calculate other items as a percentage of the base. The most common base amount used in a vertical analysis of an income statement is net sales.
This is because net sales represents the total revenue generated by a company. Other items such as selling and administrative expenses, income from operations, and cost of goods sold can be expressed as a percentage of net sales.