"Pigs and chickens" is an analogy used in the Scrum software development model to define the type of role an attendee can play at a daily scrum meeting.
In rugby, scrum means "restart the game." For programmers, a daily scrum is a short meeting at the beginning of the day where team members take stock of where they are on a project and determine what needs to be done next.
If the scrum attendee is a pig, it means he is directly accountable for completion of the task at hand. If the attendee is a chicken, he may be somewhat involved in the task at hand but is not the person whose "bacon is on the line" if the task doesn't get completed on time. At daily scrums, pigs may talk. Chickens must remain silent.
The roles, which are usually self-assigned, are intended to prevent daily scrums from going on too long and drifting off topic. In addition to enforcing the only-pigs-can-talk rule, meeting facilitators (called the ScrumMasters) will often hold scrums standing up.
The choice of labels to define the two roles is up to the team. The roles have also been described as boats and barnacles, dogs and fleas and movie stars and agents. The original inspiration for the choice of pig and chicken comes from this story:
A pig and a chicken are walking down a road. The Chicken looks at the pig and says "Hey, why don't we open a restaurant?" The pig looks back at the chicken and says "Good idea, what do you want to call it?" The chicken thinks about it and says "Why don't we call it 'Ham and Eggs'?" "I don't think so" says the pig, "I'd be committed but you'd only be involved."
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