My two cents

Choose the right metric to start with

It's all a question of the metrics that you define as important. I've made a list of 150 innovation metrics that you can take as inspiration. 

A classical way to think about the quality of a service, product or idea is to use the Venn Diagram of Design Thinking as proposed by IDEO:

But some say that creating products and services that people want, that make enough money and that can be created is also what brings us to more consumption and to the creation of more waste.

Therefore some, like those pushing for a shift from Human Centered Design to Planet Centric Design, propose to add a few more criteria:

Service Quality can be measured

The quality of a Service is, in fact something that has a formula and that some people have found ways to calculate:

"Service quality (SQ), in its contemporary conceptualisation, is a comparison of perceived expectations (E) of a service with perceived performance (P), giving rise to the equation SQ=P-E". — Wikipedia

Maybe it's not a metric

For some service, a standard metric might be too reductive to evaluate if the service is really working for its purpose. Many NPO, therefore, use a method called "Most Significant Change":

"The Most Significant Change (MSC) approach involves generating and analysing personal accounts of change and deciding which of these accounts is the most significant – and why." — Bronwen McDonald, Kaye Stevens, Theo Nabben and Patricia Rogers

Going further

To go further, listen to this 50-minute interview with Tim Dekker, a Senior Service Designer at Kiwi.com, on the question: "How to effectively measure a service".

More Service Design questions and answers like this one

Check out all the questions about showing and discovering the value and impact of Service Design.