The standard deviation is 0.8 for a single HLL, but a ratio involving 2 of them will have about 1.6%. Simple arithmetic:<p><pre><code>    conversions x (1+0.008) &#x2F; [visitors x (1 - 0.008)
    ~ (conversions &#x2F; visitors) (1 + 2 x 0.008 + ...)
    ~ (conversions &#x2F; visitors) (1 + 0.016 + ...)
</code></pre>
(It should actually be a bit less if you do the calculation carefully, probably 1.6&#x2F;sqrt(2).) 12kb is also a pretty big filter, but I guess reasonable.<p>It&#x27;s definitely true that non-statistical errors are highly likely to drown this out. But why have extra error if you don&#x27;t need it?