This strategy was developed to take advantage of short-term swings in the price spread between Brent and WTI (represented in the trading pair BNO and USO). The idea was that the spread would deviate from the mean over the short-run due to various macroeconomic, international policy, transportation/logistics, military conflicts, national crises, and a myriad of other far-reaching issues, but that in the long-run the spread would return to the mean.
We choose an entry point of positive or negative 1.8 standard deviation where we would go long one of the assets and short the other to gain positive alpha return while the spread returns to zero. Since the spread is calculated BNO minus USO (since BNO generally holds a premium to USO), whenever the spread reached the positive 1.8 standard deviation we should short BNO and go long USO as the spread would likely make its way back to zero (which is the mean). If the spread reached the negative 1.8 standard deviation then we would do the opposite - short USO and long BNO. The reason we chose the 1.8 standard deviation instead of a smaller or larger value is because we wanted to catch the spread on the way back to the mean (meaning we wanted a value larger than the first standard deviation), but we also wanted the algorithm to trade semi-actively so we wanted a value less than the second standard deviation. We settled on the value of 1.8 since it was closer to the second standard deviation, allowing us to catch the spread on the way back to the mean in a more active fashion without succumbing to a lot of the noise which would happen if the standard deviation value for the entry point were somewhere in the 1-1.5 mark.
In realizing that our algorithm would not be able to catch the spread right at the point where it reverted back to the mean since we only scheduled the trading function to happen once an hour in to the train day, we decided to make our exit point at the point where the absolute value of the deviation of the spread was less than 1. This way we would catch a nice profit from the spread moving back towards the mean (the journey from positive or negative 1.8 to less than positive or negative 1) while still having a nice back stop to make sure we didn't exit the positions too late after the point where the spread crossed over the mean again but in the opposite direction. This also helped manage slippage (due to the illiquidity of BNO) where we would have a nice cushion from 1 (or negative 1) back to zero while we exited BNO - which usually took most of the trading day.
You can look at the research notebook here --> https://www.quantopian.com/research/notebooks/BNO-USO%20Pairs%20Trading%20Research%20Notebook.ipynb