The grading curve
LetsVibe grades on a single site-wide bell curve computed dynamically from raw rubric scores (never stored in the database). You can switch between Bell Curve and Raw Rubric on any leaderboard or project page.
Why a curve
AI reviewers are good at comparing projects but poor at absolute scores: they hedge toward the middle. In LetsVibe's own testing, a deliberately terrible app and a carefully built one, graded by the same pipeline, averaged about 28 and 68 on a 0-100 scale, and repeated runs of the same project varied by several points. A raw score therefore says little on its own, so LetsVibe grades on a bell curve: what counts is how your project compares, first with two published reference projects and then with the other projects in its category.
How it works
LetsVibe grades on a single site-wide bell curve computed dynamically from raw rubric scores (never stored in the database). By default, a raw score of 20 anchors 0 and 90 anchors 100, with the curve centered at 75 (C) and blending toward the real site-wide average and spread as more projects are graded (fully at 30 projects). If 5 or more projects score above 90 (or below 20), the top or bottom anchor automatically shifts to the 5th-highest or 5th-lowest score so the scale adapts as coding tools improve.
- Default anchors: Raw 20 maps to 0, raw 90 maps to 100, and the prior midpoint 75 maps to 75 (C).
- Automatic scaling over time: Once 5 or more projects score above 90 (or below 20), that endpoint shifts automatically to the 5th-highest (or 5th-lowest) raw score.
- Adaptive spread: As more public projects are graded, the curve blends smoothly from the prior toward the real site-wide average and standard deviation (fully at 30 projects).
- Single global scale: Every project shares the same curve regardless of category, so Bell Curve rank order always agrees with Raw Rubric rank order.
Curved letter grades
| Grade | Curved score range (0–100) |
|---|---|
| A+ | 97–100 |
| A | 93–96.9 |
| A- | 90–92.9 |
| B+ | 87–89.9 |
| B | 83–86.9 |
| B- | 80–82.9 |
| C+ | 77–79.9 |
| C | 73–76.9 |
| C- | 70–72.9 |
| D+ | 67–69.9 |
| D | 63–66.9 |
| D- | 60–62.9 |
| F | below 60 |
Raw scores are never modified; reports and API responses always preserve the uncurved rubric score alongside the Bell Curve score.