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How the Guess Plausibility Score Works
Last updated: June 16, 2026
What is the GPS?
The Guess Plausibility Score (GPS) shows how statistically realistic each of your birth guesses was, based on real-world birth data from the CDC and ACOG. Instead of just saying "you were wrong," GPS tells you how plausible your guess was compared to what actually happens in births every year.
The GPS Buckets
| Label | GPS Range | Meaning |
|---|---|---|
| Very Plausible | >= 0.75 | Well within typical birth ranges |
| Plausible | 0.50-0.74 | Within normal variation |
| Unlikely | 0.25-0.49 | On the edges of typical ranges |
| Wild Guess | < 0.25 | Well outside typical birth ranges |
How GPS Is Calculated
BabyGuessr uses z-scores to measure how far each guess is from the real-world average for that birth attribute.
Formula
z = (guess - mean) / standardDeviation
GPS = 1 - (|z| / 3)
- A guess exactly at the average earns GPS = 1.0 (perfect plausibility)
- A guess 1.5 standard deviations away earns GPS = 0.5
- Any guess beyond 3 standard deviations earns GPS = 0 (capped)
Real-World Data Behind Each Attribute
Birth Weight
- Source
- CDC National Vital Statistics System (NVSS), "Births: Final Data for 2023"
- Mean
- ~3,300 g (7 lb 4 oz)
- Standard deviation
- ~500 g (1 lb 1 oz)
- Distribution
- Log-normal (captures right-skew in real birth weights)
Birth Length
- Source
- CDC Growth Charts (WHO-aligned)
- Mean
- ~50.8 cm (20.0 in)
- Standard deviation
- ~2.0 cm (0.8 in)
- Distribution
- Normal
Gestational Age
- Source
- CDC NVSS, "Births: Final Data for 2023"
- Mean
- ~39.0 weeks
- Standard deviation
- ~1.5 weeks
- Distribution
- Normal with slight left tail
Due-Date Offset (Days Early/Late)
- Source
- ACOG Committee Opinion No. 700 - "Methods for Estimating the Due Date"
- Mean
- 0 days from due date
- Standard deviation
- ~7 days
- Distribution
- Normal
Time of Day
- Source
- CDC NVSS natality microdata (time-of-birth distribution)
- Pattern
- Daytime peak due to scheduled deliveries
- Model used
- Flattened sinusoidal curve
- Mean
- ~14:00 (2 PM)
- Standard deviation
- ~6 hours
Why These Distributions?
These distributions were chosen because they reflect the actual statistical shape of real birth outcomes. For example, birth weight is log-normal because it has a natural floor (babies cannot weigh less than zero) and a right skew - very large babies are more common than very small ones when looking at the extremes. The CDC and ACOG sources are the gold-standard references for U.S. birth statistics.
Why GPS Makes Scoring Fair
GPS means that guessing something impossible (for example, a birth weight of 15 lbs) does not just cost you points - it earns a Wild Guess label that reflects how far outside real-world norms the guess was. Conversely, if a baby genuinely arrives at an unusual time or weight, GPS acknowledges that and your guess is still evaluated against real distributions, not penalized for the baby's individual outcome.