Typically, you'd do Measurement System Analysis (MSA)[1] which includes things like repeatability, reproducibility, drift and linearity. A standard traceable to an authority such as NIST is needed to do this.

For length, MSA can be easily conducted. For happiness, what metrology system are you going to use? What's the proposed measurement method?

[1] MSA: https://www.metrology-journal.org/articles/ijmqe/pdf/2010/02...