title: get z standardized score within each group
body: <p>Here is the data. </p>
  
  <pre><code>set.seed(23) data&lt;-data.frame(ID=rep(1:12), group=rep(1:3,times=4), value=(rnorm(12,mean=0.5, sd=0.3)))
  
     ID group     value
  1   1     1 0.4133934
  2   2     2 0.6444651
  3   3     3 0.1350871
  4   4     1 0.5924411
  5   5     2 0.3439465
  6   6     3 0.3673059
  7   7     1 0.3202062
  8   8     2 0.8883733
  9   9     3 0.7506174
  10 10     1 0.3301955
  11 11     2 0.7365258
  12 12     3 0.1502212
  </code></pre>
  
  <p>I want to get z-standardized scores within each group. so I try</p>
  
  <pre><code>library(weights)
  data_split&lt;-split(data, data$group) #split the dataframe
  stan&lt;-lapply(data_split, function(x) stdz(x$value)) #compute z-scores within group
  </code></pre>
  
  <p>However, It looks wrong because I want to add a new variable following 'value'
  How can I do that? Kindly provide some suggestions(sample code). Any help is greatly appreciated .</p>