Le 16/11/2010 15:48, Invisible a écrit :
>>> Does anybody here know enough about statistics to come up with answers?
>>
>> That's what standard deviation (σ) on my old casio FX-180P is made for.
> 
> The standard deviation tells you how variable something is. However:
> 
> 1. This, by itself, does not tell you how many measurements you need to
> take to achieve a given level of accuracy.

Does your goal of accuracy ever exists ?
How do you define accuracy ?
If you want a standard deviation/mean < value X, at least you have a
criteria to know when to stop. (but you should have a minimal number of
sample).
What if your measurement will forever be split between value A and value
B, would you still insist on getting a single value C with a standard
deviation about 0.00001% of C ?

You cannot know the number of samples before sampling, to at least get
an idea of the repartition law.

You should not apply the table of confidence for normal distribution if
your sample do not at least somehow show a pattern of normal distribution! (

Have a look at the Chebyshev's inequality in the link at the end, they
are more generic... but you get to take less risk if it's following a
normal distribution law (can you prove it without a few samples first ?)

> 
> 2. If you compute the SD from the data you gathered, then the SD itself
> may be inaccurate.

That's why there is Bessel correction for some estimators.


http://en.wikipedia.org/wiki/Standard_deviation

-- 
A good Manager will take you
through the forest, no mater what.
A Leader will take time to climb on a
Tree and say 'This is the wrong forest'.

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