It's a fact.  When it comes to Google ranking you don't have to
be a lot better to beat out the competition. 

So let's take another dip into the "every little bit helps"
pool.  

Now believe it or not there are those who like to debate the
merits of using dashes or underscores in domain names.  

Some assert dashes are better.  

Some have an ongoing love affair with underscores. 

Others are certain there is no difference.  

I agree you do get a bit of a bounce in Google if you do this
right.  But it's only marginal.  

Still let's end this debate once and for all and PROVE which is
better. 

To sort this out we need to conduct a study.  Using the Google
search results to test if Google treats dashes or underscores
the same or differently.  

The guinea pig search term I picked is "affordable search
engine placement".  

There's nothing special about it.  It's more or less your
run-of-the-mill multi-word search term.  

So let's get searching.  

First, to set a benchmark I cast the broadest net possible by
doing a simple search using 

   affordable search engine placement 

Google returned these results: 

 Searched the web for affordable search engine placement.     
 Results 1 - 10 of about 78,600 

That simply says there were 78,600 pages indexed by Google for
ANY of those keywords.  

Next I searched on the same phrase only this time I separated
the words by dashes like this: 

   affordable-search-engine-placement 

Google turned up these results: 

  Searched the web for affordable-search-engine-placement.
  Results 1 - 10 of about 1,160. 

As you can see our term with dashes gave considerably fewer
results than the one without. 

Then I searched on the same words separated by underscores: 

  affordable_search_engine_placement 

For this one Google didn't find much: 

  Searched the web for affordable_search_engine_placement.
  Results 1 - 4 of about 6. 

Whoa!  Next to no pages with underscores, right? 

Finally I searched for 

  "affordable search engine placement" 

Note the quotes.  Using quotes limits the search results to one
specific phrase.  Just like you were doing an advanced search
for that exact phrase.  

In this case Google returned these results: 

  Searched the web for "affordable search engine placement".   
  Results 1 - 10 of about 1,160. 

Huh, exactly the same number of pages as with the keyword
phrase with dashes.  

Okay so what do we got? 

Let's see.  The first search returns what you could say is a
free for all of listings with any of the words in the keyword
phrase.  That's why there are so many search results.  

SIDEBAR:  Reality check time.  This is how most search.  In
fact I saw a stat the said only 3% use the advanced search
feature provided by Google.  Yet the dramatically bigger number
of resulting SERPS explains why it is harder to rank high
sometimes.  Reason being you are going up against a whole bunch
more pages - some unrelated to what is being searched for.  So
it's takes more juice, ie on-page optimization, internal links
and maybe even in bound links to come out on top. 

Now our study also showed the phrase with underscores (which
Google treats as any other CHARACTER) produced negligible
results.  As in next to none. 

While the keyword phrase with dashes and the exact phrase
search turned up the same number of listings.  

At this point you should be wondering "Why is that?"  

Glad you asked.  Even if you didn't let me explain.  Oh and
since this is important engage your brain NOW.  

The reason for this apparent match of search results is Google
uses the dash to separate the words in the phrase.  Programmers
call this a "delimiter".  In essence Google sees it as a space
or separator between the words.  Or in other words Google
treats the dash as a spacer.  

Yet Google does NOT treat the underscore as a delimiter.  Again
to Google it's just another character.  

Which is proven by the search results.  Had Google treated the
dash and underscore alike the number of SERPs returned for 

 affordable_search_engine_placement 

or 

 affordable-search-engine-placement 

would be identical.  But as you saw they are not.  Not even
close. 

So to answer the original question of which is better dashes or
underscores is obvious isn't it?  You want to go with dashes in
your domain names, folder names, files names etc. 

That's because using dashes to separate the words will give you
the biggest Google impact -whatever that impact may be.  Since
Google can parse the different words.  While using underscores
won't help one iota.  

Look.  This isn't theory or speculation.  It's fact.  And you
can repeat the same searches with any keyword phrase you want
and you'll get the same results.  

In any case let's be real.  Don't expect some kind of massive
boost from this dash trick.  Sure it can help at tad as part of
an over all optimization scheme.  But whether or not you use
dashes in a domain, folder or file name is not going to be what
gets you top Google listings.   Content and links are. 

Still this study settles the debate about dashes and
underscores.  Giving you yet another little thing you can do to
rank well.