Say you have a team of 4. 1 junior, 2 mid-level and 1 senior.
The senior engineer can get the story completed quickly (say 1 point). For the junior, the work is more complicated; he/she doesn't know the platform as well and the expectation is that it will take 3 points for him/her.
The mid-level people can complete the story in 2 points.
The team average is 2 points for the same story.
Do you assign story points based on the assignee or based on team average?
Story points should be independent of the particular individual assigned to that story. Story points are not meant to be equated directly to implementation time. (In fact, some Scrum teams use non-numerical story size for this reason.) Therefore, the points should not change depending on who is assigned. Rather, you might say something like this:
Senior Engineer is capable of undertaking 30 story points during a sprint; Junior Engineer is capable of undertaking 10 story points in the same sprint.
Although one concept of Scrum might be that all team members are capable of doing the work on the backlog, it doesn't mean that they are all "equally" capable. By extension, it doesn't mean that all team members should undertake the same number of story points during a sprint.
The base of story point does not depend on the ability of the individual to carry out the task. Ability will reflect in the velocity with which an experience or a junior engineer will execute the story points. A senior can execute more story points where as juinor engineer will execute less. In any story point estimation it is important that the team agree to unit of work which they can define as story point and do their estimation in relation to that. If there are conflicts the approach that we usually take is to discuss and agree on certain story point based on the discussion that is carried out with pros and cons.
