How to do Activity Duration estimation

After identifying the resources available for the activities of the project, the project manager can estimate how much time the activity will take to complete. The project manager can take the help of the resource calendar, resource requirement and other existing company processes. The estimator needs to be giving as much as information as possible to get a realist estimation to complete the activity.
Following methods are used to estimate the duration an activity might take.
One-Time estimating: This kind of estimation is based on expert judgement of estimator. This estimate is best when it is made by the resource who will work on the activity. Estimator looks at the historic performance on similar activities and makes her judgement.
One of the biggest problems with this estimation is the estimator estimates some duration for the activity and then adds some buffer time to be on the safe time. This can hurt the reputation of the project manager if the planned activities are completed much before the time. Since the estimations are made by an expert or the boss in many cases, there is no buy-in from the project team. This might impact the performance of the team and they might miss the baselines for the activity.
Analogous estimating: It is top-down approach and very similar to expert judgement but the estimate is derived from the duration for the similar activities or the projects. Though ideally this approach can be used for activities and project both, practice is to use for the project by senior management or the project sponsor. It is the responsibility of the project manager to estimate duration for the activities using some sound techniques.
Parametric estimating: when exact information for the activity is not available, the project managers can use regression analysis and learning curve methods to estimate duration for the activities. In regression analysis two variables are plotted on the scattered graph and their relationship is derived using mathematical modelling. Learning curve method says that any resource will take less time than she took last time to finish a similar activity.
Heuristics: It is just a rule of thumb. After working on large number of similar activities, one knows how much time it takes for a similar activity to complete. This can be derived from the results of the parametric estimation.
Three point estimation: This method believes that there is very small probability that a project will be completed on a given date estimated in the beginning of the project. This method takes into account the most likely cases of finishing a project. The project manager can come up with best, worse and most likely duration for finishing an activity. A simple average of these estimates can be used as best estimate for the activity. The project manager can also use more complex weighted mathematical formula to derive duration for any activity.