Cause and Effect Diagram
The cause and effect diagram is also referred to as the Ishikawa diagram (created by Kaoru Ishikawa) or the fishbone diagram. In the cause and effect diagram, the problem or effect is stated on the head and the 5M (men (people), machine, material, measurements and methods) along with environment become the bones of the diagram. The team would brainstorm a list of possible causes for the problem and placed the causes in one of the six categories. The cause and effect diagram can assist the team in identifying areas that are contributing to the problem in a structured manner.
Design of Experiments (DOE)
One way to determine whether something is the root cause that is affecting a process would be to test it out in an experiment. For example if the team believes centrifuging stat and routine samples together contributes to the problem of increased turnaround time (TAT), then the team could design an experiment to test this out. Centrifuges could be designated as "stat centrifuges" and the TAT with and without the use of dedicated stat centrifuges could be compared. Keep in mind that just like in a scientific experiments, other factors must be controlled to validate the finding of the experiment. While DOE is a good way to verify root cause, it can be impractical or costly to setup.