A team is looking to create KPIs to measure the success of stretch assignments for employee growth and development.  Examples include providing opportunities for employees and leaders to present to higher-level leaders and creating opportunities for employees to be a part of projects they may not normally have exposure to.  How can the team measure the success of these development opportunities?

Survey participants after stretch assignments. 
Promotion rates for the group that participates in the stretch assignments versus company overall promotion rate may be another option. 

- What skills will this assignment create or grow?
- Who or what metrics are impacted by assignment completion?

In my experience as an HR professional, employees believe in the efficacy of programs like this when they (and their peers) are selected for internal promotional opportunities.  My suggestions, from a pragmatic perspective, are the following:
1.  Maintain a list of employees who are participating in these opportunities
2.  Use the organization's applicant tracking system to be aware of when one of these employees applies for an internal job posting.  If the posted position is ~70% fit with the employee's career skills and interests, encourage the hiring manager to (at a minimum) invite the employee to a first-round interview.  If it becomes evident during the interview that the employee lacks knowledge skills or experience that are important to success in the position, candidly share this feedback with the employee and coach them on ways to gain the required qualifications.  The same guidance applies for employees who apply for posted positions that they are not qualified for.  These actions could easily be incorporated into a metric that can be measured and tracked (ie. number and % of employees receiving feedback on their applications for posted positions).  
3.  Create and maintain a list of positions that could be potential growth opportunities for these employees.  If it should become obvious that an external candidate must be hired for a particular position, (ie. external candidate possesses industry-specific technical expertise), provide feedback to qualified internal candidates who are not selected.  Coach these employees on ways to gain skills that might help them get selected for a similar promotional opportunity in the future.  This could easily be incorporated into a metric that can be measured and tracked (ie. number and % of internal candidates receiving feedback when an external candidate his hired).

The team can measure the success of stretch assignments by establishing KPIs such as enhanced employee engagement, skill development, and career progression. For instance, in a case study, a technology company introduced stretch assignments allowing junior developers to lead client presentations. The KPIs included feedback from higher-level leaders, improved communication skills, and subsequent promotions. Additionally, tracking participation in diverse projects and measuring project success rates can indicate the effectiveness of providing exposure to varied roles, further contributing to the overall assessment of development opportunities.