Is talking to oneself loudly a very common sign of schizophrenia? Schizophrenia.com states that 


  Replaying or rehearsing conversations out loud- i.e. talking to yourself [is a] (very common sign) [of schizophrenia].


Looking at the site it's not clear who edits it (or what their credentials are)... so is this claim supported by research, i.e. is talking to oneself loudly a very common sign of schizophrenia?

And is it so irrespective of the content of the talk? E.g. talking to "oneself" as if having a conversation with an imaginary person [different than oneself] seem somewhat different to me (and indicative of hallucinations) in contrast with clearly self-directed self-talk, e.g. as described in the first paragraph of a NYT article of someone wondering out loud in a supermarket. Or is it simply the case that so few people talk to themselves loudly so that the group (of loud self-talkers) is dominated by those who have schizophrenia? (About 75% of people with schizophrenia experience auditory hallucinations.)

I also found that 


  Self-talk [and by this the article means loud] and behavior rituals were found to be common behavioral features in adults with Down syndrome, and in most instances, were not a sign of a psychiatric disorder.


But then the number of people with Down syndrome is only about 1/12 of those with schizophrenia.
Before I answer this question, I would like to point out that at the bottom of Schizophrenia.com is a disclaimer, and part of it says


  No health information on Schizophrenia.com, including information about herbal therapies and other dietary supplements, is regulated or evaluated by the Food and Drug Administration and therefore the information should not be used to diagnose, treat, cure, or prevent any disease without the supervision of a medical doctor.


If you look at their write-up on the DSM diagnostic criteria, it seems that they are taking about the diagnosis criterion of Disorganised Speech within the Diagnostics and Statistics Manual (DSM).

The bit after the brackets:


  these are also called "word salads". Ongoing disjointed or rambling monologues - in which a person seems to talking to himself/herself or imagined people or voices.


is their own annotation to what is in DSM-IV (American Psychiatric Association, 1994)



The key point is the bit where they say that the talking is "ongoing disjointed or rambling monologues".  Just talking to yourself is not necessarily disorganised speech.

WebMD describes disorganised speech in their schizophrenia page.


  They may act silly and giggle for no apparent reason. They often make up words and sentences that make no sense to other people. And they often don't show facial expressions.
  
  Examples of disorganized speech include:
  
  
  Making up words (neologisms). For example: "I'm going to the park to ride the wallywhoop."
  Rhyming words (clang speech). For example: "Deck the halls with boughs of holly, folly, polly, dolly, hello Dolly, want a lollipop?"
  Saying sentences that make no sense to other people (word salad). For example: "Give paper floor me school hop bus."
  Repeating exactly what someone else has said (echolalia).
  


Another point of note is that whether following DSM-IV or the latest edition — DSM-5 (American Psychiatric Association, 2013) — disorganised speech on its own is not enough to diagnose schizophrenia.  It would have to accompany at least one more item from Criteria A, plus there are also Critera B through to F to meet.

References

American Psychiatric Association, 1994. Diagnostic and Statistical Manual of Mental Disorders (Fourth Edition). Washington, DC: American Psychiatric Publishing.

American Psychiatric Association, 2013. Diagnostic and Statistical Manual of Mental Disorders (5th Edition). Washington, DC: American Psychiatric Publishing.