If driving instructors were given a star rating rather than the simple A or B gradings the differences would be much easier to understand.
The gradings are awarded to instructors who have reached the bare minimum standard of competence to be allowed to join the profession. Those who score from 1-12 marks above the minimum are awarded a grade B while those who score from 13-21 above the minimum are awarded a grade A.
So those who attain a grade B would get between 1-3 stars for their teaching. Although this is seen as good enough to teach people to drive it is not seen as being good enough to enable them to join the official register of driving instructor trainers, ORDIT.
This is because their ability is seen as not being of a high standard and the DVSA are trying to stop these people passing on their poor teaching skills to others.
Those who are grade A would get 4 or 5 stars. This grades is seen as being of a higher standard. Naturally 5 stars would be better than 4. However instructors of this quality account for only around 5% of the total.
So in a town that has 20 instructors you would only expect to find one of these. The vast majority would have a star rating of 1,2 or 3 as around 70% of instructors are grade B.
Given the choice, when reading reviews, most people tend to pick products and services that have a rating of better than 3, even though those that are more highly rated have a higher cost. This is because people realise the benefits of improved reliability.
Research has also shown that when people are made aware of the grading of instructors in this star rating manner, the vast majority spend more time seeking out a grade A 4 or 5 star instructor.
So if you want your driving skills to be better than average you would do well to take your time carrying out the proper research. Always ask an instructor what their grade is and specify that you will need to see their DVSA 'A' grading report as part of the contract for taking lessons.