I apologise for perhaps the confusing title, I'll try and elaborate a little better.
Many discussions I see surrounding password entropy focus on the specific context of the range of choices available for that nominated data. In isolation this is fair enough, but it seems to ignore the possibility of choosing that range of data in the first place. Perhaps an example would better illustrate.
In another very recent question, a bible verse was considered to have drawbacks for a password, as there are (apparently) ~33,000 bible verses. This leads to an entropy of 16 bits from my understanding. When looking at say a case insensitive latin alphabet password, it has 4.7 bits of entropy per symbol, so if you compared entropies a 4 digit upper / lower case password (4.7 * 4 = 18.8 bit) password should be 'harder' to guess than a randomly chosen bible verse.
Wouldn't it be fair to say though that a password is much more likely to under go a brute force attack as opposed to be checked as valid bible verses? Is not the 4 digit password significantly weaker than a bible verse? Can we ever judge the entropy of a password ignoring the liklihood of the data range selection? I'm just curious as to how valid it is to say password XYZ is bad because there isn't much variety in a datasets, given that it's extremely unlikely that an attacker will be able to narrow down the dataset used for password choice so specifically.
Sorry security newbie, but it just struck me as odd that someone may use entropy to justify why something such as a long bible verse may be a bad password.