title: creating z-scores for subset of rows
body: <p>I am new to R programming and having trouble with some basic code.</p>
  
  <p>I have a dataframe with following columns: condition(factor), user(factor) and sensitivity(int). For each user there are 20 sensitivity items. I need to create a new column with standardized sensitivity score for each user. This way I can compare sensitivity scores across users. I am having trouble writing a loop (or something similar) that can create z-scores for each item per user.</p>
  
  <p>I have 14 unique users (this will increase) with 20 sensitivity items. </p>
  
  <p>Thanks.</p>