Terry Richards wrote:
Your measurable output is whatever you bill for divided by how often you bill.
For translating that would be something like source words per month. For interpretting, it would be days worked per month.
Performance indicators are a little more difficult to define. It should be something that demonstrates how well you are doing the job.
For translation, if your work is proof-read by another person and the results are shown to you then it could be errors detected per thousand words or something like that. If, as is often the case, you get little or no concrete feedback on the quality of your work, then you will have to use something more vague like how often you get work from the same customer.
I have no idea what you could use for interpretting!