### Towards optimal experimentation in online systems

by CHRIS HAULK It is sometimes useful to think of a large-scale online system ( LSOS ) as an abstract system with parameters $X$ affecting responses $Y$. Here, $X$ is a vector of tuning parameters that control the system's operating characteristics (e.g. the weight given to Likes in our video recommendation algorithm) while $Y$ is a vector of outcome measures such as different metrics of user experience (e.g., the fraction of video recommendations resulted in positive user experiences). If we wish to tune the system parameters $X$ for optimal performance of $Y$, there are several challenges: The relationship between $X$ and $Y$ may be complex and poorly understood It may be impossible to simultaneously maximize every element of $Y$, requiring trade offs There may be hard constraints on the $X$ and $Y$, either individually or in combination, that limit what we deem to be acceptable operating points for the system One approach to this problem is to experiment with one or two system p