Bollinger Bands

What it computes

An SMA of closes with bands StdDev population standard deviations above and below it.

It describes bars that have already printed. QuantZ does not claim this indicator has predictive value, and it is not a recommendation to do anything.

Inputs and lookback

• Length · default 20
• StdDev · default 2

Reads the close of each loaded bar. With the default settings the first value is on bar 20; earlier bars are left empty rather than padded.

What it assumes about the series

• Bars arrive oldest to newest at one interval, each with a finite open, high, low and close. The input is the chart’s loaded series, whatever its length.
• The function is given bars, not sessions: it does not know the timeframe, where a session opens, or that a gap in the tape is a gap.
• Warm-up bars are omitted rather than padded. The first value appears once the arithmetic has enough history, and nothing is drawn before it.
• While the newest bar is still forming, its value uses the live close and moves until the bar closes.

Whether a drawn value can change

forming-bar · Moves only on the forming bar. The newest bar’s value moves with its live close until that bar closes. Appending later bars leaves closed-bar values unchanged. Loading older history or correcting input bars can change them; each entry states its history assumptions.

This class is read from the registry entry and checked by a test against the function itself: for a class that says closed bars are final, appending a bar must leave every earlier point unchanged, and for a class that says they are not, it must not.

Outside its inputs

• Nothing outside the loaded bars: no longer history than the chart has loaded, and no other interval.
• Nothing inside a bar: no individual trades, no order book, no bid or ask.
• No other symbol, no benchmark, no news, no fundamentals, no earnings dates.
• Volume is not read at all.