(Since critiquing comes in many styles, and beginners might get confused as to their particular uses and strengths.)
The best critiques deal with a little bit of both, and for the experienced critiquer it really depends on personal style and how they feel like doing things. However, there are certain instances which demand a line crit and others in which it's inappropriate.
When to do a line-by-line style critique:
-- in prose, if there are more than a few passages that you want to comment on, fix errors, or rewrite. This can be a good lesson for people who's sentence structure is a little bit funky or a story where style and usage is very important and needs to be commented as such
-- in essays, going by paragraph or sentence to point out arguments and counterarguments.
-- in poetry, when there are small details to comment on. Might go by line or just by stanza/reasonable chunk and interpret. These sorts of peicemeal interpretations are useful for extended or very detailed works. The same goes for general critiquing.
When not to do a line critique:
-- when, in any format, the writer is so novice and the piece so full of errors that pointing out every singe one of them would lead to unhappiness and many wasted red pens. In this situation, it's better to just write a few paragraphs about what generally needs to improve and how the writer can make that general improvement.
-- work with very few errors; if you can't find more than five in prose/a very long poem, then there's no point in quoting the whole thing to point those out. Quote the relevant pieces and then sum them up and say generally what was good about the piece at the end; interpretations, and general opinions are also extremely helpful. Nothing is more frustrating than to see your entire story quoted and think that you got this great line-by-line critique, only to find that it's mostly your work taking up the space, while the critiquer gets away with doing a minimal amount of work.
examples:
good line crit 1 [poetry]
good line crit 2 [prose]
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