Is it truck factor or bus factor?

Question

It seems that both these terms get thrown around a lot. Both, I think, describe the same idea. Which was established first?

Also, it seems like some people describe it as a good thing to have a low x factor, while others describe it as a good thing to have a high x factor. Which is it?

Answer 1

You want a high truck/bus factor:

Truck Factor (definition): "The number of people on your team who have to be hit with a truck before the project is in serious trouble"

(From: http://www.agileadvice.com/archives/2005/05/truck_factor.html)

i.e. you don't want parts of the code that only one person knows how it works or only one person can extend/maintain. Knowledge should be spread amongst the whole team via things like wiki info and pair-programming.

Answer 2

The principal is the same, whether you call it:

• bus number
• truck number
• bus factor
• truck factor
• et al

Also, the principal is the same whether or not you describe it using a higher number as being better, or a lower number being better:

• A high bus number is good if you are describing the number of project members who could be hit by a bus and have the project survive;
• A low bus number is good if you are describing the number of project members who survive a bus crash and have the project survive.

I did look into it once upon a time, but I don't recall which came first (see @Paolo's answer). Regardless of which came first, I have experienced enough confusion about it that I make sure all parties are using the same version of the number, high or low. ;)

Answer 3

The Truck Number (or Truck Factor) is the number of people with key knowledge that you cannot replace i.e. if that number of people went simultaneously under a truck you wouldn't be able to carry on developing.

I believe that certain, German chemical companies forbid key members of staff from travelling together for this very reason...

Discussion here: http://c2.com/cgi/wiki?TruckNumber

Here's a story about Bill Atkinson being the one key person in the Mac's truck factor - one of the key people that worked on QuickDraw during the early days of the Mac. Had a car accident apparently and people were concerned that he wouldn't be able to finish his work on the Mac's graphics software:

http://folklore.org/StoryView.py?project=Macintosh&story=I_Still_Remember_Regions.txt

A high truck number is better - i.e. it's harder to wipe out that many critical people at once...

A low truck number is worse - i.e. there is a greater risk that a few critical people could be ill, or leave or die, leaving the project in a state of unrecoverable collapse.

Pair progamming is a good way to distribute critical knowledge around a team so that the truck number is as high as possible.

Answer 4

Wikipedia says bus number is "more commonly known as truck number" But in the US, "hit by a bus" is practically an idiom, while "hit by a truck" is not (although either phrase is easily understood.) Regarding high/low being good, the wikipedia article says:

"High bus numbers are good (with the best case being equal to the number of developers on a project). This means all developers understand the codebase more or less equally. A low bus number (especially, one) represents a high risk."

I'd add to what @cartoonfox said: Promiscuous pair progamming is a good way to distribute critical knowledge around a team so that the truck number is as high as possible. If you don't swap pairs often and with many different team members, knowledge isn't distributed very quickly.