Data Scrub Skill

Garbage in → backtest lies. Before ANY strategy runs on new data, audit it.

Ask for

• File path
• Expected timeframe (1m, 5m, 1h, 1d)
• Expected timezone (UTC recommended)
• Expected date range
• Symbol(s) covered

Run these checks — each must PASS

1. Schema

• Required columns: timestamp, open, high, low, close, volume
• Dtypes correct? (timestamp = datetime64, OHLCV = float)

2. Timestamp integrity

• Monotonic increasing, no duplicates
• No gaps greater than 1 bar (for 24/7 crypto) or expected gaps (market hours for equities)
• Timezone explicit and consistent — never naive
• No daylight-saving jumps

3. Bar integrity

• low <= open <= high for every bar
• low <= close <= high for every bar
• low <= high (sanity)
• volume >= 0

4. Missing data

• Count NaN in each column
• Flag any bar where volume = 0 AND OHLC all equal → likely a stale / synthetic bar
• Flag bars where high == low with non-zero volume → suspicious

5. Outliers

• Return series: count bars with |return| > 20% (flag for review)
• Volume spikes > 20x 30-bar rolling median → real or data error?

6. Symbol-level (if multi-symbol)

• Symbol universe consistent across time, or point-in-time?
• Delistings handled?
• Ticker changes / splits adjusted?

7. Corporate actions (equities)

• Dividend-adjusted?
• Split-adjusted?
• If not, warn user before using for backtests.

Code template

import pandas as pd

def scrub(df: pd.DataFrame, timeframe: str = "1h") -> dict:
    issues = {}
    expected_delta = pd.Timedelta(timeframe)

    issues["dupes"]       = df.index.duplicated().sum()
    issues["monotonic"]   = df.index.is_monotonic_increasing
    issues["gaps"]        = ((df.index.to_series().diff() > expected_delta).sum())
    issues["ohlc_break"]  = ((df["low"] > df["open"]) |
                             (df["low"] > df["close"]) |
                             (df["high"] < df["open"]) |
                             (df["high"] < df["close"]) |
                             (df["low"] > df["high"])).sum()
    issues["neg_volume"]  = (df["volume"] < 0).sum()
    issues["nan_rows"]    = df.isna().any(axis=1).sum()
    issues["stale_bars"]  = ((df["volume"] == 0) &
                             (df["open"] == df["close"])).sum()
    return issues

Output format

Single markdown table with Check / Result / Severity / Fix.

Severity:

• BLOCK — do not backtest until fixed
• WARN — acknowledge and document
• OK — passed

End with a one-sentence verdict: ready for backtest / fixes required.