title: Why numbers are sometimes treated as characters in R?
body: <p>I have created a panel data set by compiling data from several sources. But why the variables, i.e. <code>local_aus</code>, <code>hyv_aus</code>, and <code>hyv_aman</code> are being considered as characters instead of numbers? I used: 
  <code>mutate(local_aus = as.numeric(local_aus)</code>, <code>hyv_aus = as.numeric(hyv_aus)</code>,<code>hyv_aman = as.numeric(hyv_aman))</code></p>
  
  <p>However, R shows,  <code>warning messages: NAs introduced by coercion</code>. But why are those numeric values being considered as charcaters? </p>
  
  <pre><code>Classes ‘tbl_df’, ‘tbl’ and 'data.frame':   856 obs. of  24 variables:
   $ district             : num  11704 10408 11921 12007 11313 ...
   $ year                 : num  1970 1970 1970 1970 1970 1970 1970 1970 1970 1970 ...
   $ local_aus            : chr  "178145" "94390" "119375" "56375" ...
   $ hyv_aus              : chr  "3010" "850" "2095" "3785" ...
   $ broadcast_aman       : num  70325 9435 33340 1495 316580 ...
   $ local_transplant_aman: num  673060 270550 282655 35825 188655 ...
   $ hyv_aman             : chr  "3185" "920" "3080" "820" ...
   $ local_boro           : num  6450 12050 41430 14450 45970 ...
   $ hyv_boro             : num  67930 10630 121340 15640 116500 ...
   $ danger_days_aus      : num  0 0 142 4 108 434 5 36 33 1 ...
   $ benefit_days_aus     : num  0 0 9 0 21 110 0 0 0 0 ...
   $ danger_days_aman     : num  0 0 32 0 43 218 0 0 29 2 ...
   $ benefit_days_aman    : num  0 0 89 0 110 426 3 52 53 2 ...
   $ danger_days_boro     : num  0 0 1 0 0 0 0 0 0 0 ...
   $ benefit_days_boro    : num  0 0 0 0 0 0 0 0 0 0 ...
   $ abovemax_aus         : num  2 25 1 37 4 18 29 19 45 42 ...
   $ belowmin_aus         : num  1 1 2 4 2 0 3 3 2 0 ...
   $ abovemax_aman        : num  0 0 0 0 1 0 2 1 1 6 ...
   $ belowmin_aman        : num  0 0 0 0 0 0 0 0 0 0 ...
   $ abovemax_boro        : num  2 7 0 10 1 8 4 7 5 12 ...
   $ belowmin_boro        : num  116 123 107 92 76 115 138 125 124 89 ...
   $ rain_aus             : num  5969 1088 6902 5637 3831 ...
   $ rain_aman            : num  5477 650 5806 2291 2900 ...
   $ rain_boro            : num  601.6 38.1 1067.3 381 387.4 ...
  </code></pre>