What is a denominator?

Any rate, a citation rate, a block rate, a share of voice, is a fraction, and the denominator is what's underneath the line, the full group being counted. A block rate of 16% only means something once you know what the 16% is out of: all domains contacted, all domains that returned a readable response, all domains in some registry. Swap the denominator and the same underlying data produces a different headline number.

This is where a lot of quoted statistics quietly go wrong. If a crawl includes sites that timed out, returned an error, or blocked the crawler itself, and those failures get dropped from the denominator rather than counted as "status unknown," the resulting percentage describes only the domains that were reachable, not the full population anyone assumed it covered. Excluding the unreadable domains isn't dishonest by itself, but reporting the rate without saying what was excluded is.

The honest version of any rate states its denominator plainly: not "16% of domains block a crawler" but "16% of the 33,670 domains with a readable robots.txt block a crawler," with the tens of thousands of unreadable domains named as excluded rather than folded silently into either side.