Re: [PERFORM] Bad n_distinct estimation; hacks suggested?

["Andrew Dunstan" <andrew(at)dunslane(dot)net>]

Josh Berkus said:
>
>
> Well, unusual distributions are certainly tough.  But I think the
> problem  exists even for relatively well-distributed populations.
> Part of it is, I  believe, the formula we are using:
>
> n*d / (n - f1 + f1*n/N)
>
[snip]
>
> This is so broken, in fact, that I'm wondering if we've read the paper
> right?   I've perused the paper on almaden, and the DUJ1 formula
> appears considerably  more complex than the formula we're using.
>
> Can someone whose math is more recent than calculus in 1989 take a look
> at  that paper, and look at the formula toward the bottom of page 10,
> and see if  we are correctly interpreting it?    I'm particularly
> confused as to what "q"  and "d-sub-n" represent.  Thanks!
>

Math not too recent ...

I quickly read the paper and independently came up with the same formula you
say above we are applying. The formula is on the page that is numbered 6,
although it's the tenth page in the PDF.

q = n/N  = ratio of sample size to population size
d_sub_n = d = number of distinct classes in sample

cheers

andrew