---
license: mit
language:
  - en
tags:
  - text-classification
  - medical
  - nhs
  - clinical-letters
  - distilbert
pipeline_tag: text-classification
---

# NHS Medical Letter Classifier

Fine-tuned **DistilBERT** (`distilbert-base-uncased`) for classifying OCR'd NHS medical clinic letters into 49 letter type categories.

## Model Details

| Parameter | Value |
|---|---|
| Base model | `distilbert-base-uncased` |
| Training samples | 13,672 |
| Classes | 49 |
| Epochs | 6 |
| Batch size | 16 |
| Learning rate | 2e-5 |
| Max sequence length | 512 tokens |
| Cleanlab corrections | 212 labels relabeled (1.6% of dataset) |

## How We Got Here: Experiment Journey

### 1. Baseline: TF-IDF + LinearSVC
- **Approach:** TfidfVectorizer (unigram+bigram, 50k features) with CalibratedClassifierCV(LinearSVC)
- **Result:** ~91% accuracy on the original label set
- **Takeaway:** Strong baseline, but limited by bag-of-words representation

### 2. Label Merging (Critical Improvement)
- **Approach:** Consolidated synonymous labels (e.g., "Nephrology" to "Renal", "Minor Illness Consultation" to "Pharmacy") and dropped ambiguous/administrative labels
- **Result:** Accuracy jumped from ~91% to ~96%
- **Takeaway:** Label quality matters more than model architecture. Reduced label set from ~51 to 49 meaningful categories

### 3. DistilBERT Baseline (Our Core Model)
- **Approach:** Fine-tuned `distilbert-base-uncased`, 4 epochs, 512 tokens, 70/10/20 stratified split
- **Result:** Top-1: 95.76% | Top-3: 98.06% | Top-5: 98.61%
- **Takeaway:** Strong performance, established as the baseline for all further experiments

### 4. ClinicalBERT & BioClinicalBERT
- **Approach:** Tested domain-specific models (`medicalai/ClinicalBERT`, `emilyalsentzer/Bio_ClinicalBERT`)
- **Result:** Similar to DistilBERT (~95-96%), no meaningful improvement
- **Takeaway:** General-purpose DistilBERT captures enough for this task; domain pre-training didn't help

### 5. Longformer (1024 tokens)
- **Approach:** `allenai/longformer-base-4096` at 1024 tokens with global attention on CLS, case-sensitive
- **Result:** Comparable to DistilBERT at 512 tokens
- **Takeaway:** Most discriminative information is in the first 512 tokens; longer context doesn't help

### 6. Hierarchical Architecture
- **Approach:** Two-stage: DistilBERT body for CLS embeddings, per-clinic LogisticRegression heads. 51 fine labels mapped to 25 broad categories
- **Result:** Did not outperform flat DistilBERT
- **Takeaway:** The flat classification space works well; hierarchical routing adds complexity without benefit

### 7. LLM Relabeling (GPT-5-mini)
- **Approach:** Used OpenAI Batch API to get GPT-5-mini to reclassify all 13,672 samples. Trained DistilBERT on LLM-assigned labels
- **Result:** 86.22% vs original labels | 93.24% vs LLM labels (Top-1)
- **Takeaway:** LLM agrees with original labels ~85.7% of the time. LLM labels are different but not better — the original clinical labels carry domain knowledge the LLM lacks

### 8. Consensus Relabeling
- **Approach:** Only change labels where both BERT and GPT-5-mini agree the original label is wrong
- **Result:** Only 4 out of 9,569 samples met the consensus criteria
- **Takeaway:** BERT memorizes its training labels, so it almost never disagrees with originals on training data. Consensus is too strict

### 9. Soft Knowledge Distillation
- **Approach:** Got GPT-5-mini top-5 predictions with confidence scores as soft labels. Trained with blended loss: alpha * CE(hard) + (1-alpha) * KL(soft || student), alpha=0.5
- **Result:** Top-1: 95.32% (-0.44pp) | Top-3: 97.48% (-0.58pp)
- **Takeaway:** LLM self-reported confidence scores are too noisy/uniform. Soft KL loss stayed flat at ~3.5. Would need actual logprobs for this to work

### 10. Cleanlab: Remove Mislabeled Samples
- **Approach:** Confident learning (Northcutt et al. 2021). 3-fold cross-validation for out-of-sample probabilities, then `find_label_issues()` to detect mislabeled samples. Removed 142 flagged training samples and retrained
- **Result:** Top-1: 95.90% (+0.14pp) | Top-3: 97.70% (-0.36pp)
- **Takeaway:** Small top-1 gain, but removing ambiguous samples hurt ranked predictions. Manual inspection confirmed ~99% of flagged samples were genuinely mislabeled

### 11. Cleanlab: Relabel Instead of Remove
- **Approach:** Same cleanlab detection, but replaced wrong labels with model's predicted label instead of removing samples
- **Result (vs original test labels):** Top-1: 95.80% | Top-3: 97.92% | Top-5: 98.46%
- **Result (vs corrected test labels):** Top-1: 98.06% | Top-3: 99.09% | Top-5: 99.38%
- **Takeaway:** The ~2pp gap between original and corrected evaluation reveals that the remaining "errors" are mostly test set noise, not model mistakes. True model performance is ~98% top-1

### 12. Production Model (This Model)
- **Approach:** Fresh 3-fold cleanlab on the **entire** dataset (13,672 samples). Found 212 mislabeled samples (1.6%), relabeled all. Trained on full corrected dataset for 6 epochs
- **Sanity check:** 99.74% accuracy on training data (expected, since model saw all data)
- **Estimated true accuracy:** ~98% top-1, ~99% top-3 based on corrected-label evaluation

## Key Findings

1. **Label quality > model architecture.** Label merging (+5pp) and cleanlab corrections (+2pp true accuracy) had more impact than any model change
2. **DistilBERT is sufficient.** Domain-specific models (ClinicalBERT, BioClinicalBERT) and longer context (Longformer) didn't help
3. **~1.6% of labels are wrong.** Discharge summary (9.1%), Paediatrics (7.2%), and Physiotherapy (6.8%) are the noisiest classes
4. **The model is better than naive metrics suggest.** When evaluated against corrected labels, top-1 jumps from ~96% to ~98%

## Labels (49 classes)

- `A&E`
- `Ambulance Notification`
- `Audiology`
- `Bowel Cancer Screening`
- `Breast Clinic`
- `Cancer Screening`
- `Cardiology`
- `Colposcopy`
- `Dermatology`
- `Diabetes & Endocrine`
- `Diet Services`
- `Discharge summary`
- `ENT`
- `Echocardiogram`
- `Elderly Care`
- `Gastroenterology`
- `General Surgery`
- `Genetics`
- `Haematology`
- `INR`
- `Immunology`
- `Mammogram`
- `Maternity`
- `Maxillofacial`
- `Mental Health`
- `Neurology`
- `Neurosurgery`
- `Obstetrics & Gynaecology`
- `Oncology`
- `Ophthalmology`
- `Orthopaedics`
- `Out of Hours`
- `Paediatrics`
- `Pain Management`
- `Pharmacy`
- `Physiotherapy`
- `Plastic Surgery`
- `Radiology`
- `Renal`
- `Respiratory`
- `Retinal Screening`
- `Rheumatology`
- `Sexual Health`
- `Speech and Language Therapy`
- `Stroke Services`
- `Urgent Care Centre`
- `Urology`
- `Vascular`
- `Walk in Centre`

## Usage

```python
from transformers import AutoTokenizer, AutoModelForSequenceClassification
import torch, json

model = AutoModelForSequenceClassification.from_pretrained("mansour94/kynoby-william-bert-classifier")
tokenizer = AutoTokenizer.from_pretrained("mansour94/kynoby-william-bert-classifier")

# Load label map
from huggingface_hub import hf_hub_download
label_map = json.load(open(hf_hub_download("mansour94/kynoby-william-bert-classifier", "label_map.json")))
id2label = {int(k): v for k, v in label_map["id2label"].items()}

text = "Dear Dr Smith, I am writing to inform you about the patient's ophthalmology appointment..."
inputs = tokenizer(text, return_tensors="pt", truncation=True, max_length=512)
with torch.no_grad():
    logits = model(**inputs).logits
    probs = torch.softmax(logits, dim=-1)

# Top-3 predictions
top3 = torch.topk(probs, 3)
for i in range(3):
    idx = top3.indices[0][i].item()
    conf = top3.values[0][i].item()
    print(f"  {id2label[idx]}: {conf:.1%}")
```