Information Technology Reference
In-Depth Information
Don't use probabilities to describe small sets of observations. Success in two of five
cases does not mean that the method “works 40% of the time”. The percentage gives
the result an authority it might not deserve.
Units of Measurement
Two quantities are commonly measured in computer science: space and time. For
time, the basic units are the second (s), minute (min) and hour (h); note that it
is unusual to give the abbreviated forms of these units.
2
For the divisions of the
second—the millisecond (ms or msec), microsecond (
sec), and nanosec-
ond (ns or nsec)—some readers may be unsure of the notation. For example, “ms”
might be interpreted as
microsecond
. State such units in unabbreviated form at least
once. When writing about hours or minutes use a colon rather than a stop to separate
the components of the time. That is, write “3:30min” rather than “3.30min”.
For space, the basic units are bit and byte. These are usually combined in tenth
powers of 2 rather than third powers of 10,
3
although, confusingly, in IEEE standards
it is powers of 10 that are correct (Table
9.2
).
If there is any likelihood that, for example, a reader could interpret “Mb” as
megabit
, use “Mbyte” or “megabyte” instead. The larger units, especially “Pb”,
“Eb”, “Zb”, and “Yb”, are unfamiliar to most readers and should be written in full
at least once.
There are few derived units in computing other than the transfer rate of bytes per
second, as in “18Mb/sec”. It was surprising to see “millibits” in a paper on arithmetic
coding (in which symbols can be represented in a fraction of a bit). The unit is so
unusual that “thousandths of a bit” is preferable.
Choose units that are easy to understand. For example, seconds can be preferable
to minutes because fractions of a minute can be confusing: does “1.50min” mean
one
and a half minutes
or
one minute and fifty seconds
? (This problem can be avoided
by using colons instead of stops, as discussed above.) Also, as values such as clock
speeds and transfer rates are quoted in seconds, use of minutes makes comparison
more difficult. On the other hand, “13:21 h” is perhaps kinder to the reader than
“47
µ
sor
µ
10
3
s”.
4
Some units, although in general use, are not well-defined. For example, MIPS
(a million instructions per second) and gigaflops (a billion instructions per second)
are increasingly meaningless; they cannot be used to compare machines of different
.
8
×
2
Though variations of these units are common, with “sec” for seconds, for example, and “hr” for
hours. While SI usage is to be encouraged, consistency is essential; choose an abbreviation and
stick to it.
3
Except, apparently, when the capacity of disk drives is being discussed. For example, a megabyte
of disk is 10
6
000 bytes, or somewhat less than a megabyte or 2
20
=
1
,
000
,
=
1
,
048
,
576 bytes of
memory.
4
As an engineer might write it. Or “4
10
4
s”, as it might be written by a mathematician.
.
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×
