Which of the following is considered a 'key performance indicator' (KPI) in sales?

The concept of a 'key performance indicator' (KPI) in sales refers to quantifiable measures that are used to gauge a company's performance over time, specifically in relation to achieving its sales goals. The average deal size closed is a critical metric because it directly impacts overall revenue and provides insight into customer purchasing behavior. By analyzing the average deal size, sales teams can gauge the effectiveness of their sales strategies, identify trends, and forecast future revenues more accurately.

In contrast to the correct choice, total number of customer complaints may provide important feedback regarding customer satisfaction and service quality but does not directly reflect the sales performance or revenue outcomes. The number of social media followers serves as an indicator of brand awareness and engagement but does not directly correlate to actual sales figures. The amount spent on advertising is a measure of marketing investment but does not directly reflect sales success or effectiveness. Thus, the average deal size closed stands out as a direct and relevant KPI in assessing sales performance.