I like to think it in this way:
Self improvement/Personal development information and materials doesn't fail. It's the person who reads the material that doesnt learn the lesson.
Picture personal development as a biology text book. Now picture a student who is suppose to study it for an exam. The student takes the biology exam, he/she will either pass or fail. The student will also get a grade on how well the student knows the material. The test or exam is like a reference point indicating whether the student learned something or not.
Now, you can memorize all the PD and SI informations and you might even be able to recite them on top of your head. Does that tell you that you learned the lesson?... well, the answer is no. You have memorized it, but you haven't learn the lesson. You see, PD and SI exam or test is not like a written test, it is more like a driving exam... a performance test more to say. To say that you learned PD and SI, you need to pass the performance exam.
So how do you know if you passed the PD & SI exam when there are no exam teachers to give you a score?
Well, it's your results
that will tell you if you learned something or not, if you passed the exam or not. You see, if you learned something you will change. The change will be an evident result that you can see in your everyday life. Your habit would have change.
If your habit hasn't change, can you really say you learned something?
If you haven't learned the lesson, was it you who failed or was it the text book (PD & SI) that failed?
(a side note: on a given average in any class 0-10% will get an A+.
In PD & SI, if you get below an A- you will most likely to revert back to your old habits... But of course,... you can always take the exam as much as you like until you get it right