German tank problem
There is some practical relevance to software development here.  One shouldn't expose sequential IDs (a.k.a. serial numbers) to the public for anything non-public.I see this Hacker News post has a numerical ID in the URL, for example; I can estimate the size of Hacker News given enough of these numbers...  More directly, I can modify that numerical ID to crawl Hacker News.Many sites do this; it's generally better to generate a (random or hashed or generated from a natural key) 'slug' to use as the key instead.  For example, Amazon generates a unique, non-sequential, 10-digit alphanumeric string for each item in their catalog.