📊 Kappa Value [Neurosurgery Wiki]

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====== 📊 Kappa Value ======

The **Kappa value** (Cohen’s Kappa) is a **statistical measure of interrater agreement** for **categorical data**, adjusted for the amount of agreement that could occur by chance.

It quantifies how consistently two or more observers classify the same items beyond random coincidence.

===== 🧠 Key Characteristics =====
  * Ranges from **–1 to 1**:
    * **1** = Perfect agreement
    * **0** = Agreement equivalent to chance
    * **< 0** = Worse than chance (systematic disagreement)
  * More robust than simple percent agreement because it accounts for **expected agreement** due to randomness.

===== 📈 Interpretation Guide =====
| Kappa (κ) Value  | Strength of Agreement           |
|------------------|----------------------------------|
| < 0              | Poor (less than chance)          |
| 0.01 – 0.20      | Slight agreement                 |
| 0.21 – 0.40      | Fair agreement                   |
| 0.41 – 0.60      | Moderate agreement               |
| 0.61 – 0.80      | Substantial agreement            |
| 0.81 – 1.00      | Almost perfect agreement         |

===== 📌 Example =====
If two neuroradiologists independently evaluate MRI scans for presence of DVA and reach the same conclusion 76% of the time, but much of that agreement is expected by chance, the **Kappa** might only be **0.51**, indicating **moderate agreement**.

===== ✅ Best Practice =====
  * Use Kappa to **assess reliability** of diagnostic tools or observer consistency.
  * Always report Kappa with **confidence intervals** and **p-values** for context.
  * For ordinal scales, consider using **weighted Kappa**.