How do I determine a measurement circuit accuracy specification?

[RDXx]

Lets say that I have built my own DMM.
How do I determine the measurement accuracy for the different channels (voltage measure AC/DC, current measure with external CT's)?

A theoretical calculation taking into consideration R-, OP-, ADC-errors will not take external electrical disturbance into consideration.

If I should measure it in the real world I need perhaps measure at different temperature/scales and use a large(?) number of different individuals in order to get statistical data.

How do the established DMM company's determine their accuracy?

[TiN]

You calculate all error contributors and factors of all components in signal path, and then you can determine total uncertainty.
Main factors to start with would be input impedance, input signal dynamic range of interest, referene stability, switching leakage/charge injection, protection circuitry error contribution, ADC performance and calibration gain/offset accuracy.