Here are a few questions that could help you address this goal.
How much of my product's code is covered by unit tests?
How many defects are in my product?
Once you've asked your questions, the next step is to identify your metrics.
So, if we were to take the question of how many defects are in my product?
We could approach this in a few ways.
One metric could be the number of times a user store is rejected during
acceptance testing before it's finally accepted.
This could be represented as a percentage of rejections to acceptances across
the whole project.
You could say that a high ratio of rejections to acceptances is an indicator
that something in your process should change, so
that you can reduce the defects.
To know what a high ratio is, you might consider comparing this number
across projects that worked well versus those that didn't.
Another metric that addresses this question is the number of bugs that
are placed in your product backlog at any given time.
If you're seeing an influx of bugs in your backlog, this might indicate that you need
to review your techniques to catch defects earlier.