[Lynne]

If you look at the Advanced Search page you'll see a checkbox for whether you want the results returned as Thread or Posts:

<fieldset class="fieldset">
				<legend>Show Results as</legend>
				<div style="padding:3px">
					<label for="rb_showposts_0"><input type="radio" name="showposts" value="0" id="rb_showposts_0" checked="checked" />Threads</label>
					<label for="rb_showposts_1"><input type="radio" name="showposts" value="1" id="rb_showposts_1"  />Posts</label>
				</div>
			</fieldset>

So, it looks like the variable you want is "showposts" and you want to set it to "0"