<s>
The	O
bus	B-General_Concept
factor	I-General_Concept
is	O
a	O
measurement	O
of	O
the	O
risk	O
resulting	O
from	O
information	O
and	O
capabilities	O
not	O
being	O
shared	O
among	O
team	O
members	O
,	O
derived	O
from	O
the	O
phrase	O
"	O
in	O
case	O
they	O
get	O
hit	B-General_Concept
by	I-General_Concept
a	I-General_Concept
bus	I-General_Concept
"	O
.	O
</s>
<s>
It	O
is	O
also	O
known	O
as	O
the	O
bus	B-General_Concept
problem	I-General_Concept
,	O
truck	B-General_Concept
factor	I-General_Concept
,	O
or	O
bus/truck	O
number	O
.	O
</s>
<s>
Personnel	O
must	O
be	O
both	O
key	O
and	O
irreplaceable	O
to	O
contribute	O
to	O
the	O
bus	B-General_Concept
factor	I-General_Concept
;	O
losing	O
a	O
replaceable	O
or	O
non-key	O
person	O
would	O
not	O
result	O
in	O
a	O
bus-factor	O
effect	O
.	O
</s>
<s>
The	O
term	O
was	O
first	O
applied	O
to	O
software	O
development	O
,	O
where	O
a	O
team	O
member	O
might	O
create	O
critical	O
components	O
by	O
crafting	O
code	O
that	O
performs	O
well	O
,	O
but	O
which	O
also	O
is	O
unavailable	O
to	O
other	O
team	O
members	O
,	O
such	O
as	O
work	O
that	O
was	O
undocumented	O
,	O
never	B-Application
shared	I-Application
,	O
encrypted	O
,	O
obfuscated	O
or	O
not	O
published	O
.	O
</s>
<s>
The	O
"	O
bus	B-General_Concept
factor	I-General_Concept
"	O
is	O
the	O
minimum	O
number	O
of	O
team	O
members	O
that	O
have	O
to	O
suddenly	O
disappear	O
from	O
a	O
project	O
before	O
the	O
project	O
stalls	O
due	O
to	O
lack	O
of	O
knowledgeable	O
or	O
competent	O
personnel	O
.	O
</s>
<s>
The	O
expression	O
"	O
hit	B-General_Concept
by	I-General_Concept
a	I-General_Concept
bus	I-General_Concept
"	O
describes	O
a	O
person	O
either	O
dying	O
or	O
more	O
generally	O
disappearing	O
suddenly	O
from	O
the	O
project	O
.	O
</s>
<s>
Team	O
members	O
do	O
not	O
literally	O
have	O
to	O
get	O
"	O
hit	B-General_Concept
by	I-General_Concept
a	I-General_Concept
bus	I-General_Concept
"	O
for	O
the	O
"	O
bus	B-General_Concept
factor	I-General_Concept
"	O
to	O
apply	O
—	O
any	O
number	O
of	O
events	O
could	O
occur	O
in	O
which	O
a	O
team	O
member	O
could	O
be	O
suddenly	O
and	O
substantially	O
prevented	O
from	O
working	O
on	O
the	O
project	O
.	O
</s>
<s>
If	O
all	O
5	O
people	O
who	O
know	O
how	O
to	O
bake	O
disappear	O
,	O
then	O
the	O
team	O
cannot	O
produce	O
bread	O
,	O
so	O
the	O
team	O
's	O
bus	B-General_Concept
factor	I-General_Concept
is	O
5	O
.	O
</s>
<s>
There	O
is	O
a	O
rare	O
alternative	O
definition	O
for	O
the	O
bus	B-General_Concept
factor	I-General_Concept
,	O
namely	O
:	O
the	O
number	O
of	O
people	O
who	O
are	O
indispensable	O
for	O
the	O
project	O
.	O
</s>
<s>
In	O
other	O
words	O
,	O
it	O
is	O
the	O
minimum	O
number	O
of	O
people	O
who	O
are	O
a	O
single	B-Architecture
point	I-Architecture
of	I-Architecture
failure	I-Architecture
.	O
</s>
<s>
If	O
using	O
this	O
definition	O
,	O
then	O
a	O
high	O
bus	B-General_Concept
factor	I-General_Concept
is	O
considered	O
a	O
bad	O
thing	O
(	O
since	O
the	O
loss	O
of	O
any	O
person	O
included	O
destroys	O
the	O
project	O
)	O
,	O
and	O
zero	O
is	O
considered	O
the	O
ideal	O
bus	B-General_Concept
factor	I-General_Concept
.	O
</s>
<s>
An	O
early	O
instance	O
of	O
this	O
sort	O
of	O
query	O
was	O
when	O
Michael	O
McLay	O
publicly	O
asked	O
,	O
in	O
1994	O
,	O
what	O
would	O
happen	O
to	O
the	O
Python	B-Language
language	I-Language
if	O
Guido	O
van	O
Rossum	O
were	O
hit	B-General_Concept
by	I-General_Concept
a	I-General_Concept
bus	I-General_Concept
.	O
</s>
<s>
"	O
Truck	B-General_Concept
number	I-General_Concept
"	O
was	O
already	O
a	O
recurring	O
concept	O
in	O
the	O
Organizational	O
Patterns	O
book	O
published	O
in	O
2004	O
,	O
itself	O
an	O
evolution	O
of	O
the	O
work	O
published	O
in	O
the	O
first	O
book	O
of	O
the	O
Pattern	O
Languages	O
of	O
Program	O
Design	O
series	O
in	O
1995	O
,	O
which	O
was	O
the	O
publication	O
record	O
of	O
the	O
first	O
Pattern	O
Languages	O
of	O
Programs	O
conference	O
in	O
August	O
1994	O
,	O
where	O
it	O
was	O
referenced	O
in	O
patterns	O
including	O
Solo	O
Virtuoso	O
.	O
</s>
<s>
Studies	O
conducted	O
in	O
2015	O
and	O
2016	O
calculated	O
the	O
bus/truck	O
factor	O
of	O
133	O
popular	O
GitHub	B-Application
projects	O
.	O
</s>
<s>
The	O
results	O
show	O
that	O
most	O
of	O
the	O
systems	O
have	O
a	O
small	O
bus	B-General_Concept
factor	I-General_Concept
(	O
65%	O
have	O
bus	B-General_Concept
factor	I-General_Concept
≤	O
2	O
)	O
and	O
the	O
value	O
is	O
greater	O
than	O
10	O
for	O
less	O
than	O
10%	O
of	O
the	O
systems	O
.	O
</s>
<s>
In	O
many	O
software	O
development	O
projects	O
,	O
one	O
goal	O
is	O
to	O
share	O
information	O
in	O
order	O
to	O
improve	O
the	O
bus	B-General_Concept
factor	I-General_Concept
,	O
potentially	O
to	O
the	O
size	O
of	O
the	O
entire	O
team	O
.	O
</s>
<s>
A	O
good	O
bus	B-General_Concept
factor	I-General_Concept
means	O
many	O
individuals	O
know	O
enough	O
to	O
carry	O
on	O
and	O
the	O
project	O
could	O
still	O
succeed	O
even	O
in	O
very	O
adverse	O
events	O
.	O
</s>
<s>
Several	O
ways	O
to	O
improve	O
the	O
bus	B-General_Concept
factor	I-General_Concept
have	O
been	O
proposed	O
:	O
</s>
