title: R dataframe: setting values in columns to NA based on them being less than a predetermined number
body: <p>given the following data frame below:</p>
  
  <pre><code>&gt;head(ethn)    
      caucasian   asian       latino
      8.28E-27    1.00E+00    1.00E+00
      7.23E-01    6.22E-01    7.48E-01
      2.73E-01    1.60E-01    1.00E+00
      1.09E-04    3.16E-01    4.74E-02
      7.99E-01    6.80E-01    7.37E-01
      1.00E+00    1.00E+00    1.00E+00
      1.00E+00    1.00E+00    1.00E+00
  </code></pre>
  
  <p>How do I set all values less than 0.30 in all columns to NA?
  It is a much larger dataframe but I am just giving this as a short example.
  thanks in advance.</p>