To estimate PBIs you need to choose an average story and assign it a number of points (for example 5). Then you compare all other stories to that story and assign points accordingly. My question is, are you really supposed to compare every single story to that original story from the beginning of the project? If not, then how do you make sure your scale remains more or less consistent throughout the project?
Thanks.
Story points are subjective; always. What a '5' means to you may mean something else to another team member. As your team grows and gains more experience after each sprint, you get an understanding of how the rest of your team members view a '5'. Every sprint, you'll get a better understanding, and therefore a better estimate.
There is no need to make sure your scale remains consistent, it will happen naturally.
You could compare it to average tasks from the last sprint. You don't need to utilize that "base-line" story every single time, especially as your team evolves.
Story point is a measure of the unit-of-work's EFFORT + COMPLEXITY + RISK. It is a relative value and is baselined for a specific team. It will and should not vary by each team member because irrespective of the experience and skill level of a team member, a 2 story-point user-story is twice as big as a 1 story-point (and so-on). Comparing it with tasks or time is not a good idea - as it does not take the difference in skill level of the team members.
I recommend teams who are new to Agile or wish to have meaningful discussions around story points to establish a story-point baseline based on historic/well-defined user-stories. Sort these stories based on its effort and complexity (risk is usually situational) and establish the relative scale by choosing the lowest ranked user-story as a 1 story-pointer. This will help the team understand compare and have meaningful discussions in refinement meetings.
