Quality Filtering: Separating Signal From Noise

If you scrape the open web, most of what you get is spam, SEO-generated junk, parked domains full of ads, auto-translated machine output, and boilerplate. Training a model on this raw mess makes a worse model than training on a smaller but cleaner subset.

Common filter techniques

1. 1Language ID (keep only target languages)
2. 2Length filters (drop pages under 50 words)
3. 3Character-level rules (drop pages with too much punctuation)
4. 4Repetition filters (drop pages where one line repeats 50 times)
5. 5Perplexity filters (use a small LM to score how natural the text looks)
6. 6Classifier-based filters (train a model on Wikipedia vs. random web)

The perplexity trick

Train a small language model on a known-good corpus like Wikipedia. Run it on any web page and compute its perplexity, a measure of how surprising the text is. Random spam has very high perplexity (unpredictable garbage). Genuine writing has low perplexity. Keep only the low-perplexity pages.

Classifier filters

Take a sample labeled as high-quality (curated lists, Wikipedia, reference books) and another sample of random web text. Train a binary classifier. Then run it on the full corpus and keep the pages it labels high-quality. GPT-3 famously used this approach with a simple logistic regression.

# Pseudocode for a simple quality classifier
from sklearn.feature_extraction.text import HashingVectorizer
from sklearn.linear_model import LogisticRegression

# Labels: 1 = high quality, 0 = random web
X_train = [...]  # list of documents
y_train = [...]  # 1s for curated, 0s for random

vec = HashingVectorizer(n_features=2**18)
clf = LogisticRegression()
clf.fit(vec.transform(X_train), y_train)

# Score a new document
score = clf.predict_proba(vec.transform(['some new page']))[0, 1]
if score > 0.7:
    keep_this_page()

The big idea: quality filtering is where small labs and big labs actually differ. Anyone can download Common Crawl. Not everyone can filter it well.