That's a neat method of picking a random selection. But for statistical purposes, if you have to visit every record in order to generate your sample, you might as well just apply the statistical process to the entire set and forget random sampling.

If you seek to a random position within the file and then read 2 records discarding the first. the second will always be a complete record. That way, you can pick a 100 or 1000 element sample without visiting millions of records. But as the records are variable length, they won't all have the same chance of being picked.

So then the question becomes:

  1. How much affect does the variability of length have upon the statistical validity of the sample?

    Can this be bounded such that the statistics remain (or become) valid?

  2. How many records should you pick?

    Given a huge file with variable length records, you don't know how many it contains. But as you gather the sample, you can more and more accurately estimate that number statistically.

    Can that be used to determine the size of the sample required?

  3. Can the affects of the variable length be compensated for?

    Can you use statistics applied to the sample gathered so far, to adjust the sampling process to correct its validity?

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In reply to Re^2: Random sampling a variable length file. by BrowserUk
in thread Random sampling a variable record-length file. by BrowserUk

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