The Education Minister announced some months ago that he would be asking HMIE to measure performance in a more "discriminating" way, beyond the current levels of very good, good, fair and unsatisfactory.
A new top score of "excellent" or outstanding will be added, along with a mark of "adequate", ranked between good and weak, to indicate that "strengths just outweigh weaknesses".
"The introduction of the six-point scale will enable inspectors to make more focused and refined evaluations," an HMIE guide to the new system states. "This will enhance the capacity of HMIE to recognise and acknowledge excellence, identify and share best practice on a national basis, while continuing to highlight areas where weaknesses exist."
Schools, however, will still be able to use the old four-point scale for self-evaluation purposes.
To win the accolade of "excellent", a school or authority must be a model for others. "Pupil experiences and achievements will be of a very high quality. An evaluation of 'excellent' will represent an outstanding standard of provision which will exemplify very best practice and is worth disseminating beyond the school.
"It will imply these very high levels of performance are sustainable and will be maintained."
An "adequate" rating "will indicate that pupils have access to a basic level of provision. It represents a standard where the strengths have a positive impact on pupils' experiences. However, while the weaknesses will not be important enough to have a substantially adverse impact, it "will imply that the school should take action to address areas of weakness while building on its strengths."
"Fair" and "unsatisfactory" will continue to be used to describe circumstances where pupils' education is being adversely affected to varying degrees.
The new scale will apply from August and will also cover community education.