It depends on the context! For example, imagine that you see that people are leaving within the first year.

That means we're failing to do something within that first year. During your research, you might talk to people who point to onboarding as a problem. So you investigate onboarding. How are you doing it now? What do people think about it? How are the goals set in that first year? Survey people who joined the last 3 months and gather their experience.

Your goal is to find out the real pain points that are causing the problem. In this case, you might find out that they lacked manager support. That's okay. It's important that you understand what link is broken.

Once you understand what broken link, you can create short surveys that address peoples experience with that link. An NPS survey is a simple place to start.

Medium: 3 simple steps to create a free NPS survey and an interactive results dashboard

Let's say you're at a 2/10 now. Then the KPIs you want to track, your success criteria, could be, "In 2 quarters, we want to get to a 5/10".

Set time to analyze the KPIs.

All KPIs depend on the context. While there are no must haves, there are a few general KPIs that you should think about.

Talent retention and engagement are two big ones. If your stakeholders don't track these already, it'll be up to you to convince them why those KPIs are important to track. Explain the business impact they make and what's in it for them. For example, if you're pitching talent retention, explain: "Here's where we are now, and if you don't invest, more people will leave, and here is the consequences of that. This is how much it cost to get a new hire, here is how much it costs to onboard them, etc."

It's on you to create a business case to change their mind.