Many scientific disciplines rely on "Experimental Design" to study various types of systems. Experimental design refers to a planned approach to experimentation that tries to provide statistical evidence to the outcome of experiments. The networking community rarely relies on such approaches, especially for real protocol implementations. Many improvements to protocols like TCP, including the recently proposed Multipath TCP, have been evaluated by considering a relatively limited set of simulations or experiments. Multipath TCP increases the goodput of a data transfer by simultaneously using multiple interfaces. It also improves load balancing thanks to dedicated congestion control. By applying experimental design, we conduct a large set of measurements inside Mininet with the Linux kernel Multipath TCP implementation, to measure its bandwidth aggregation and load balancing. Thanks to the experimental design approach, we are able to highlight several limitations of this implementation. We identify heuristics that lead to lower than expected performance and propose improvements.