title: R survey package: svyby + svymean: one vs many variables
body: <p>Let's assume the a data set mydata with the variables foo1..foo20 which are factors with the labels "Easy" and "Difficult". Now let's consider this code:</p>
  
  <pre><code>library(survey)
  svd &lt;- svydesign(ids = ~ 1, weights = ~ weight, data = mydata)
  svyby(~ foo1, by = ~ group, svd, svymean)$foo1Difficult
  svyby(~ foo1 + foo2 + foo3 + ... + foo20, by = ~ group, svd, svymean)$foo1Difficult
  </code></pre>
  
  <p>Are the results supposed to be identical? Is there a reason why the results could differ? Why does it make a difference whether I iterate over each variable or use all variables at once?</p>