title: Calculating Confidence Intervals for two datasets
body: <p>Quite the number of questions I've made today.</p>
  
  <p>I'd like to calculate the <strong>Confidence Interval</strong> (99% level, not 95) <strong>for the mean value of variable age</strong> of two dataframes, <code>infert_control</code> and <code>infert_patient</code> where:</p>
  
  <pre><code>infert_control = subset(infert$age, infert$case == 0)
  infert_patient = subset(infert$age, infert$case == 1) 
  </code></pre>
  
  <p><code>infert</code> is a built-in R dataset, for those not familiar with it, here it is: <strong>case 0</strong> denominates the control-group patients, <strong>case 1</strong> the actual ones.</p>
  
  <pre><code>&gt; infert
      education age parity induced case spontaneous stratum pooled.stratum
  1      0-5yrs  26      6       1    1           2       1              3
  2      0-5yrs  42      1       1    1           0       2              1
  3      0-5yrs  39      6       2    1           0       3              4
  4      0-5yrs  34      4       2    1           0       4              2
  5     6-11yrs  35      3       1    1           1       5             32
  6     6-11yrs  36      4       2    1           1       6             36
  7     6-11yrs  23      1       0    1           0       7              6
  8     6-11yrs  32      2       0    1           0       8             22
  9     6-11yrs  21      1       0    1           1       9              5
  10    6-11yrs  28      2       0    1           0      10             19
  11    6-11yrs  29      2       1    1           0      11             20
  ...
  239   12+ yrs  38      6       0    0           2      74             63
  240   12+ yrs  26      2       1    0           1      75             49
  241   12+ yrs  31      1       1    0           0      76             45
  242   12+ yrs  31      2       0    0           1      77             53
  243   12+ yrs  25      1       0    0           1      78             41
  244   12+ yrs  31      1       0    0           1      79             45
  245   12+ yrs  34      1       0    0           0      80             47
  246   12+ yrs  35      2       2    0           0      81             54
  247   12+ yrs  29      1       0    0           1      82             43
  248   12+ yrs  23      1       0    0           1      83             40
  </code></pre>
  
  <p>What would be the correct way to solve this? </p>
  
  <p>I've already calculated the Mean value of column <code>age</code> for both <code>infert_control</code> and <code>infert_patient</code>, plus the standard deviation of each subset.</p>