Effective writing is writing that produces the consequence(s) desired.

Barring a few exceptions (including writing-to-self, humor, and satire), a writer must appear to be credible (in the eyes of the target audience reading the writer's text) in order to produce the consequence(s) desired.

• If the writer does not appear to be credible,
  then why would a target audience choose to achieve the desired consequence?

Credibility & Writing-to-Others

In the context of writing-to-others, a writer appears to be credible (in the eyes of the target audience) when that writer appropriately presents:

• primary research information, and/or
• secondary research information.

Because appropriate presentation varies among discourse communities,

• a writer needs to be (or needs to appear to be) literate in the discourse conventions valued the target audience (for a particular genre/category of writing).

Hence, performing research to identify things the target audience values (e.g., the preferred discourse conventions in the context the audience is being written to) is a good idea.