Managing part-time resources

As a project manager, you 'play the hand you are dealt' in terms of managing resources, among other things like scope, cost and schedule. The ideal scenario for the project manager is to have the right resource available at the right time, and be 100% productive from the get-go. Several project managers make this assumption when they are planning work, and then have a rude-awakening when the actual output/progress is slower than expected, often times because the productivity is much lower. This is even more apparent, when a team is comprised of some full-timer's and some part-timer's. There are couple of key assumptions that need to be addressed in managing part-time resources on the project:
1) Say it takes 8 hours to perform a job by 1 person, which requires some thinking and then doing. It is a bad assumption to make that if the same person works on the job in say two 4 hour chunks, on two different days, the work will be complete.
2) Full-time resources on the project that need help from part-time resources, assume that part-time resources are available as and when needed. This again, is a bad assumption

Part-time resources are working on more than one project, and hence multi-tasking. We have often heard people say that they multi-task very well. However, several studies have been conducted to prove that wrong. Put this in context of a resource working part-time on two projects, in order to switch between performing tasks on two different projects, a 'switching time cost' is incurred. Depending on the skills required, sometimes the switching cost can be significant. Consider this: Say a project resource is doing a mundane support task one week, and doing project work the next. He is thus available 50% of the time to the project. Say a resource starts working on a task in week 1, does not finish it in week 1, and then has to start working on it in week 3 (because in week 2 he is doing a mundane task), how realistic is it to assume that in week 3, the resource will lose no-time and have the same productivity as he had when he was working in end of week 1? So a task taking 80 hours (2 weeks), more often than not will not be completed in 2 weeks in the above mentioned scenario

As a project manager you need to plan for some buffer when utilizing part-time resources. Unfortunately, its not an exact science, because different individuals will have different 'switching time cost', so to plan for it requires some judgment. Also, it is a good idea to get a better understanding of vacation time or any other time off the project, especially in case of part-time resources. It is the project manager's responsibility to set expectations of the full-time resources on the project that they need to work with their part-time counterparts to plan the work better for optimal productivity.