Institutional Cost Line — foreign cost basis estimated from official flows, open methodology
"Where is the foreign cost basis?" is a daily question on Taiwan stock forums — but institutions never publish it. We build a volume-weighted estimate from official daily institutional net flows × closing prices over a rolling window, with the methodology fully disclosed and explicitly labelled an estimate — not a mystery "smart money cost" number.
Methodology and limits
Over the last N days (default 60), we take sessions with positive foreign net buying and volume-weight the closing prices by net-buy lots, returning the estimated cost line plus the latest close and its deviation. Deterministic: same data, same result. Honest limit: actual institutional fills are not public — this is a transparent approximation, not ground truth.
A cost line is not a support level
"It bounces when it drops below the foreign cost basis" has no mechanical basis. Institutions cut losses and rotate positions like anyone else, and an estimated average tells you nothing about where the next bid sits.
The line is a way to measure relative position on a reproducible basis. It is not a support guarantee and not advice.