Project management is largely about managing risks–technical risks, end-user risks, schedule risks, etc. The least risky approach is to execute project activities in series. For example, a project may comprise of tasks A, B, C, and D. The lowest risk for this project is:
Usually a project will have limitations or “rules” regarding what can be done in parallel and what can’t. This is especially true with medical device product development. For example, design inputs and acceptance criteria must be defined prior to execution of design verification.
I’m managing a product development project for a medical device start-up. The product involves a disposable, software, and hardware–all of which are being done in parallel. We are also building a quality system in parallel. It has been challenging and is very risky. If one item slips, then all are compromised.
And this has happened a few times. Are the project risks worth taking this path? For this project, it’s too late. We now have to develop contingencies to get things back on track.
I “inherited” this project mid-stream. Had I started project management at the beginning, I probably would not have planned everything in parallel. Food for thought for the next project. 
