Optimal design of observational studies: overview and synthesis
  We review typical design problems encountered in the planning of observational studies and propose a unifying framework that allows us to use the same concepts and notation for different problems. In the framework, the design is defined as a probability measure in the space of observational processes that determine whether the value of a variable is observed for a specific unit at the given time. The optimal design is then defined, according to Bayesian decision theory, to be the one that maximizes the expected utility related to the design. We present examples on the use of the framework and discuss methods for deriving optimal or approximately optimal designs. 
Juha Karvanen, Jarno Vanhatalo, Kari Auranen, Sangita Kulathinal and   Samu M\"antyniemi
math.ST stat.ME stat.TH