This is a hard question to answer but in general the questions on Step 1 is harder. More blocks, longer questions, more material. Step 1 will incorporate material from the most recent pubmed publications while the CBSE has questions that can be several years old. If you compare the newest NBMEs to the older NBMEs, they are much harder as well. These are retired questions, but they are retired for a reason - the exam has evolved over the years. Compare NBME 1 and NBME 24 and the style is different as well as NBME 13 and NBME 24.
This is anecdotal, but I know a person who took multiple CBSEs proctored through NBME for his med school (Lucky that his med school paid for these) and took his final CBSE with his highest score on it a week before Step 1 and scored 20 points lower. That doesn't mean it's a completely linear relationship and that he probably had a test geared poorly towards his strengths, but its correlation is not exactly perfect because its a predictor and not the same as Step 1.
Here you go. There was a retrospective study released in the journal a few months ago. It's an interesting read and their conclusion was that the CBSE is a weak predictor of step 1 performance. The study does say people do 15 points better on step 1, but part of this is confounded heavily by the amount of medical school a program does, like if they do MS1 and MS2, the amount of dedicated study time for step 1, the amount of preparation a student already had before taking the CBSE (medical school curriculum in dental school), studying for relatively similar material a second time and various other factors.
Also, they will calibrate the step 1 annually to that cohort around June/July to a new set of questions and readjust the curve which they do not do for the CBSE. A much larger number of med students take the step 1 so that calibration is more normalizing. But Step 1 is going pass fail, so the big conclusion to take from that paper is the CBSE doesn't really predict if you'll pass or fail step 1 - people who fail the CBSE pass Step 1. (Again this does not lead to any conclusions about the difficulty)
Although no statistically significant difference in the USMLE Step 1 pass rates was found between OMS residents with a translated passing CBSE score and those with a translated failing CBSE score, a weak positive correlation was noted between CBSE and USMLE Step 1 scores. These data do not...
pubmed.ncbi.nlm.nih.gov
