Hello Robin,

in Ratio model, we calculate ratio series of two stock price series (Ri = Xi/Yi). Standard deviation is calculated using N last Ri values - let's call it Si.

We also calculate moving average series of Ri - let's call it Mi. Then we calculate spread series Di = Ri-Mi. We call it "delta" and you can see it in ratio model backtest charts.

Z-score Zi is then calculated as Zi = Di/Si.