In several blogs, we postulated that an autoregressive (AR) model can produce more reliable measures of comparable company profit ratios (operating margin over revenue or profit rate over assets) than the naive profit model prescribed by the OECD transfer pricing guidelines. We prefer to work with profit margins because they are pure numbers, unlike profit rates over assets of different vintages. Here, we show the fixed-point equilibrium and the variance of an AR(1) model allowing the computation of a comparable profit ratio interval to benchmark related party transfers of goods and services.