title: R function ordistep for variable selection
body: <p>The first problem I have come across is I need to select variables for a RDA in R and using two different functions (<code>forward.sel</code> from <em>pack.for</em> and <code>ordistep</code> from <em>vegan</em>). I get two different results even if I'm using the same parameters. The second problem is <code>ordistep</code> returns varying results from time to time when I enter the same code line. The function <code>forward.sel</code> always returns the same selection of 3 variables whereas the function <code>ordistep</code> returns 1, 2 or 3 variables that are changing each time I re-enter the same code line. </p>
  
  <p>Specifically, I'm using data on plant root productivity and a variety of environmental conditions and trying to see which environmental conditions influences productivity parameters. </p>
  
  <p>When using <code>forward.sel</code> I get soil's volumetric water content, pH and soil C, </p>
  
  <pre><code>test=forward.sel(variables[,1:3], #productivity variables#
                   variables[,4:length(variables)], #environmental variables#
                   nperm = 999, alpha = 0.1)
  </code></pre>
  
  <p>When using <code>ordistep</code> I sometimes get soil P, soil N and pH, sometimes soil's volumetric water content, soil N , soil P, and pH,  or a mix of those variables in varying order.</p>
  
  <pre><code>mod0=rda(variables[,1:3]~1,variables[,4:length(variables)])
  mod1=rda(variables[,1:3]~.,variables[,4:length(variables)])
  
  test2=ordistep(mod0, scope = formula(mod1),
                 direction=c("forward"), Pin=0.1, perm.max = 9999)
  </code></pre>
  
  <p>I may be doing something wrong in selecting constrains for <code>ordistep</code> because it varies too much from time to time even when I'm using 9999 data permutations. I'd like to know if you can explain those variations in the results. </p>