To gain an accurate and comprehensive understanding of the impact of your coaching, you should use multiple sources and methods of data collection. This will help you to prevent bias, validate your findings, and capture different perspectives and dimensions of change. Self-assessment can be used to assess clients' skills, behaviors, and attitudes before and after the coaching, with tools such as the Multifactor Leadership Questionnaire (MLQ) or the Transformational Leadership Inventory (TLI). Feedback from clients' managers, peers, subordinates, or customers can be collected through interviews, focus groups, or 360-degree feedback tools. Additionally, performance can be measured with indicators such as sales, productivity, quality, or customer satisfaction. You can also use metrics such as retention, turnover, or promotion rates to measure the impact of your coaching on clients' career development. Lastly, the impact of your coaching on clients' organization or team can be measured with indicators such as culture, climate, or innovation. The Organizational Culture Inventory (OCI) and the Team Performance Assessment (TPA) are two tools that can be used to measure the effect of your coaching on organizational or team effectiveness.