title: Homoscedascity test for Two-Way ANOVA
body: <p>I've been using <code>var.test</code> and <code>bartlett.test</code> to check basic ANOVA assumptions, among others, homoscedascity (homogeniety, equality of variances). Procedure is quite simple for One-Way ANOVA:</p>
  
  <pre><code>bartlett.test(x ~ g)  # where x is numeric, and g is a factor
  var.test(x ~ g)
  </code></pre>
  
  <p>But, for 2x2 tables, i.e. Two-Way ANOVA's, I want to do something like this:</p>
  
  <pre><code>bartlett.test(x ~ c(g1, g2))  # or with list; see latter:
  var.test(x ~ list(g1, g2))
  </code></pre>
  
  <p>Of course, ANOVA assumptions can be checked with graphical procedures, but what about <em>"an arithmetic option"</em>? Is that, at all, manageable? How do you test homoscedascity in Two-Way ANOVA?</p>