Pratima, Skill Sets can be ambiguous when you are trying to apply a strict set of numbers to project duration. How does one define which skill set level will translate into a number hours or days. I think the PMBOK's guidance is correct to associate measuring effort by labor units. Then you as the Project Management can evaluate your project team's skill sets and establish in your plan, based on that evaluation, how long each effort will take. I'd like to add in here, to show inter-relationship between process groups, is where your Human Resource team building through Tuckman's ladder really comes into play. You have to know your team's strenghts and weaknesses to continually assess feasibility in making deadlines. If one member or group is weak in an area, you will have to train them to meet the level of effort required to complete the steps of the project that they are responsible for.