Placeholder and invalid values

Find sentinel values (-999, 0 used as missing, 'Unknown', blanks) and impossible values; convert them to NaN or flags. Concepts: Hidden missingness; sentinel values Project A: 'no checking account' (bad rate ~11.7%) is a real status, not missing; 'unknown/ no savings account' is genuinely unknown. Decide and document each. Project B: Pressure outliers and zero wind values; any 0 MW load hour is an error, not real demand.