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Re^2: Random sampling a variable record-length file.

by BrowserUk (Patriarch)
on Dec 27, 2009 at 16:04 UTC ( [id://814527]=note: print w/replies, xml ) Need Help??

in reply to Re: Random sampling a variable record-length file.
in thread Random sampling a variable record-length file.

I follow your meaning, but I don't think it gells with the thoery of Normal distributions & Sampling

buckets of rivets rather than CSV files. :-)

I think that the distinction is that grabing a handful from a bucket of rivets does not imply any positional correlation between the elements of the sample--they tend to mix random(ish)ly as they fall into the bucket.

Machine tools tend to wear with use, so its pretty standard practice to set-up the machine tool to operate at one end of the tolorance, so that as the tool wears, it slowly drifts towards the other end. If you took a sample entirely from the beginning of the run--or the end--then the sample would not be representative--in terms of average/mean/mode/variance--of the entire run.

But grabbing a handful from the collection hopper where they will have tended to randomly mix should be representative.

Similarly, a contiguous sequence from the beginning, end or middle of a file is probabilistically less likely to be a representative sample, than one picked at random from the entire file.

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.
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