Quick answer
NBME Form 14 is currently the most predictive self-assessment for Step 2 CK: it correlates at r = 0.92 with the real exam, with 80% of predictions landing within ±5–7 points (n = 5,039 verified outcomes). Forms 14–16 are calibrated to the current item bank; older forms (9–13) typically under-predict, so treat them as a floor rather than a target.
Understanding NBME Form-to-Step Conversions
NBME self-assessment forms (CBSSA for Step 1, CCSSA for Step 2 CK) report your performance as an Equated Percent Correct (EPC) on a 0-100 scale plus a 3-digit equivalent that approximates current Step 2 CK scoring. NBME does not publish the official conversion tables — the values you see on your report come from their internal item-response-theory equating model.
Form-specific bias matters. Across thousands of community-reported pairs, NBME 9 systematically under-predicts the real Step 2 CK score by ~10 points, while NBME 14-16 (the newest forms) are calibrated within ~3 points of actual outcomes. Our converter applies a per-form bias correction that matches each form's empirical track record.
Days-to-exam decay. Students typically score above their NBME prediction when the practice exam was taken several weeks before the real test, because of continued study and exam-day adrenaline. We apply the Tackett 2021 decay model (PMC8368818) as a soft adjustment when you tell us how far out you are.
Related reading
- Which NBME form is most accurate? — Forms 13/14, UWSA 2 and the Free 120 ranked by correlation and bias, from 5,039 verified outcomes.
- USMLE Step 2 CK score prediction guide — how to turn several practice scores into one honest prediction range.
- Calculating a Step 1 pass probability from CBSSA — what to do with an equated percent correct when there is no 3-digit score.
What NBME Actually Reports — and Why Step 1 Is Different
This is the single most misunderstood thing about “NBME score conversion.” The two self-assessment products report fundamentally different things, and only one of them needs converting at all.
| CBSSA (Step 1) | CCSSA (Step 2 CK) | |
|---|---|---|
| What you get | Equated percent correct (e.g. 73%) | A 3-digit score (e.g. 201) |
| Scale | 0–100 | 1–300 — same as the real exam |
| Probability of passing | Yes | Yes |
| Conversion needed? | Not possible — there is no 3-digit Step 1 score any more | Not to a different scale — but per-form bias correction still helps |
Why Step 1 has no 3-digit conversion.Step 1 became Pass/Fail on 26 January 2022. There is no 3-digit Step 1 score to predict any more, so NBME stopped providing one. Any site that converts your CBSSA percentage into a “predicted Step 1 score” is producing a number that appears on no score report. Use our Step 1 pass probability calculator instead — it reports the thing that actually exists.
And for Step 2 CK, your CCSSA already reports on the 1–300 scale. What it does not do is correct for the fact that individual forms run high or low against the real exam — which is exactly what the converter above applies.
The One Official Table NBME Does Publish
For Step 2 CK, NBME publishes a genuine score-to-probability table — and almost nobody uses it. Enter your CCSSA total score and read your probability of passing Step 2 CK if you test within about a week. Approximate values from NBME’s published pass-probability data:
| CCSSA score | Probability of passing |
|---|---|
| ≥ 239 | 99% |
| 230–232 | 97% |
| 225–226 | 95% |
| 218 (the passing score) | ~91% |
| 212 | 85% |
| 206 | 76% |
| 194 | 50% |
| ≤ 161 | 1% |
The insight most students miss: a CCSSA score equal to the passing score gives you roughly a 91% chance of passing, not 100%. Matching the threshold on a practice test is not the same as clearing it, because the real exam carries measurement error. If you are sitting at the line on practice, you are not safe — you are a coin-flip away from the wrong side of a 9% tail.
One date check worth doing: the Step 2 CK passing standard rose from 214 to 218 on 1 July 2025. Any pass-probability table published before that date is anchored to the old standard and will read optimistically. Ours is calibrated to 218.
NBME Predictor FAQs
Why does my actual NBME report show an Equated Percent Correct?
NBME uses EPC because it is form-equated — a 70 EPC means the same content mastery whether you took Form 9 or Form 16. Raw percent correct is not comparable across forms.
Which NBME form is the most predictive?
Forms 14-16 currently. They are calibrated against the current Step 2 CK item bank. Older forms (9-13) under-predict because they were calibrated against earlier test versions.
Can I trust offline or older NBME forms?
Yes for trajectory tracking, but expect to score above their prediction. Use older forms as a floor, not a target.