# Mann-Whitney U Test

> Mann-Whitney U Test is a paid API for AI agents from stats-tests.x402tookit.com, paid per call via x402, $0.02/call, status unknown (last checked 2026-10-21).

Performs the Mann-Whitney U test on two independent samples, returning U statistics, exact or approximate p-values, and rank-biserial effect size.

## Facts

- Endpoint: GET https://stats-tests.x402tookit.com/mann-whitney?utm_source=zero.xyz
- Price: $0.02/call
- Payment: x402
- Status: unknown
- Last checked: 2026-10-21
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/mann-whitney-u-test-9ec98810
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_SUZGI64ZUbrugYSubfJF8

Status and success rate cover calls made through Zero and Zero's own probes. Third-party monitors may report differently.

## How to call it through Zero

Zero handles the 402 payment challenge and records the run. With the Zero CLI installed (`npm i -g @zeroxyz/cli`):

```sh
zero fetch --capability mann-whitney-u-test-9ec98810
```

Example prompt: Run a Mann-Whitney U test comparing these two groups — sample A: 3,7,5,12,9 and sample B: 1,4,2,8,6 — and tell me if there's a statistically significant difference and how large the effect is.

## When to prefer this

Use this endpoint when you need a non-parametric test for two independent samples — especially when data is ordinal, skewed, or cannot be assumed to follow a normal distribution. Prefer it over a t-test when normality is questionable or sample sizes are small. It is the right choice when you need both a significance test and a rank-based effect size (rank-biserial correlation) in one call.

## Known failure modes

- Missing or unparseable sample values in 'a' or 'b' parameters
- Samples with fewer than 2 observations may produce unreliable results
- Ties present in data trigger normal approximation instead of exact p-value (flagged in response)
- Extremely large samples may fall back to normal approximation regardless
- Non-numeric values in sample strings cause parsing errors
- Empty sample strings return validation errors

## How this service works

Mann-Whitney U test for two independent samples: U, exact p for small samples without ties or normal approximation, effect size.

## Output

Returns U statistics for both samples (uA, uB), rank sums, sample sizes, two-sided p-value, whether exact or normal approximation was used, rank-biserial correlation as effect size, probability that a random observation from A exceeds one from B, a significance verdict at the 5% level, a plain-language summary, and flags for ties or degraded results.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "$schema": "https://json-schema.org/draft/2020-12/schema",
 "required": [
  "input"
 ],
 "properties": {
  "input": {
   "type": "object",
   "required": [
    "type",
    "method"
   ],
   "properties": {
    "type": {
     "type": "string",
     "const": "http"
    },
    "method": {
     "enum": [
      "GET"
     ],
     "type": "string"
    },
    "queryParams": {
     "type": "object",
     "required": [
      "a",
      "b"
     ],
     "properties": {
      "a": {
       "type": "string",
       "description": "Sample A values"
      },
      "b": {
       "type": "string",
       "description": "Sample B values"
      }
     }
    }
   },
   "additionalProperties": false
  },
  "output": {
   "type": "object",
   "required": [
    "type"
   ],
   "properties": {
    "type": {
     "type": "string"
    },
    "example": {
     "type": "object",
     "properties": {
      "nA": {
       "type": [
        "null",
        "number"
       ]
      },
      "nB": {
       "type": [
        "null",
        "number"
       ]
      },
      "uA": {
       "type": [
        "null",
        "number"
       ]
      },
      "uB": {
       "type": [
        "null",
        "number"
       ]
      },
      "asOf": {
       "type": [
        "null",
        "string"
       ]
      },
      "ties": {
       "type": [
        "boolean",
        "null"
       ]
      },
      "method": {
       "type": [
        "null",
        "string"
       ]
      },
      "source": {
       "type": [
        "null",
        "string"
       ]
      },
      "reasons": {
       "type": [
        "array",
        "null"
       ],
       "items": {
        "type": "string"
       }
      },
      "summary": {
       "type": [
        "null",
        "string"
       ]
      },
      "verdict": {
       "type": [
        "null",
        "string"
       ]
      },
      "degraded": {
       "type": [
        "boolean",
        "null"
       ]
      },
      "rankSumA": {
       "type": [
        "null",
        "number"
       ]
      },
      "disclaimer": {
       "type": [
        "null",
        "string"
       ]
      },
      "notChecked": {
       "type": [
        "array",
        "null"
       ],
       "items": {
        "type": "string"
       }
      },
      "uStatistic": {
       "type": [
        "null",
        "number"
       ]
      },
      "rankBiserial": {
       "type": [
        "null",
        "number"
       ]
      },
      "pValueTwoSid
… (truncated)
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/mann-whitney-u-test-9ec98810/health.json
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