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The tolerance may be specified as a factor or percentage of the nominal
value, a maximum deviation from a nominal value, an explicit range of
allowed values, be specified by a note or published standard with this
information, or be implied by the numeric accuracy of the nominal value.
Tolerance can be symmetrical, as in 40±0.1, or asymmetrical, such as
40+0.2/-0.1.

It is good engineering practice to specify the largest possible tolerance
while maintaining proper functionality, since closer or tighter tolerances
are more difficult to manufacture and hence cost more to either build or
buy.

Tolerance is different from safety factor, but an adequate safety factor
will take into account relevant tolerances as well as other possible
variations.

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Electrical component tolerance
An electrical specification might call for a resistor with a nominal value
of 100? (ohms), but will also state a tolerance such as "±1%". This means
that any resistor with a value in the range 99? to 101? is acceptable. It
would not be reasonable to specify a resistor with a value of exactly 100?
in any case, because the exact resistance will vary with temperature,
current and other factors beyond the control of the designer.


http://www.southwest.com.au/~jfuller/electronics/resistors.htm
Resistors are constructed to provide predetermined resistances. Most common
resistors are guaranteed to be within 5% of their marked value.
('Metal-oxide' resistors with a blue body are guaranteed to meet their
marked value plus, or minus 1%.)


http://www.tpub.com/neets/book1/chapter1/1q.htm
    Although you may find any of the above colors in the third band, red,
orange, and yellow are the most common. In some cases, the third band will
be silver or gold. You multiply the first two bands by 0.01 if it is silver,
and 0.1 if it is gold.  The fourth band, which is the tolerance band,
usually does not present too much of a problem. If there is no fourth band,
the resistor has a 20-percent tolerance; a silver fourth band indicates a
10-percent tolerance; and a gold fourth band indicates a 5-percent
tolerance. Resistors that conform to military specifications have a fifth
band. The fifth band indicates the reliability level per 1,000 hours of
operation.


http://www.mycableshop.com/techarticles/ResCodes.htm
Resistors come in standard values such as 1K, 2.2K, 4.7K, and so on.  Why
these values?  Consider a range of resistors with a 10% tolerance.  The
resistors are designed with values such that a 10% variance of one resistor
would meet or overlap with the 10% variance of the next resistance value.

To clarify, the 10% tolerance range for a 1K resistor would be 0.9K to 1.1K.
So the next resistor value going up the scale would be 1.2K since it's 10%
tolerance range would be 1.08K to 1.32K.  The lower end of the tolerance
range for the 1.2K overlaps a little with the upper end of the range for the
1K.

In the case of a range of resistors with a 5% tolerance, there would have to
be more unique values to allow for overlap.  Starting with the 1K resistor,
the next resistor value up the scale would have to be 1.1K to allow for an
overlap.  Resistors with a 20% tolerance on the other hand would have bigger
jumps between values.  Starting with the 1K resistor, the next value up the
scale would be 1.5K.  In this case, the upper value for a 1K would be 1.2K,
while the lower value for a 1.5K would be 1.2K.



http://en.wikipedia.org/wiki/Resistor
Preferred values
Standard resistors are manufactured in values from a few milliohms to about
a gigohm; only a limited range of values called preferred values are
available. In practice, the discrete component sold as a "resistor" is not a
perfect resistance, as defined above. Resistors are often marked with their
tolerance (maximum expected variation from the marked resistance). On color
coded resistors the color of the rightmost band denotes the tolerance:

  silver 10%
  gold 5%
  red 2%
  brown 1%.
Closer tolerance resistors, called precision resistors, are also available.

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5-band axial resistors
5-band identification is used for higher tolerance resistors (1%, 0.5%,
0.25%, 0.1%), to notate the extra digit. The first three bands represent the
significant digits, the fourth is the multiplier, and the fifth is the
tolerance. 5-band standard tolerance resistors are sometimes encountered,
generally on older or specialized resistors. They can be identified by
noting a standard tolerance color in the 4th band. The 5th band in this case
is the temperature coefficient.

http://www.electronic-circuits-diagrams.com/tutorials/resistors.shtml
Tolerance is the extent to which the resistor value sways from the original
value. You may think as to why the resistance value should change from the
printed value? Well, we live in a world that is far from perfect and
resistors are no exceptions. Their value changes mainly due to the change in
temperature.