title: Error in dplyr summarise_impl(.data, dots) : expecting a single value
body: <p>I want to standardize the <code>changesize</code> as the following code,and It seems to me there is no problem in my code. Why this gives me the error as</p>
  
  <blockquote>
    <p>Error in summarise_impl(.data, dots) : expecting a single value</p>
  </blockquote>
  
  <pre><code>str(pricechange_0.5_2)
  Classes ‘tbl_df’, ‘tbl’ and 'data.frame':1907600 obs. of  13 variables:
  $ url           : chr  "http://item.yhd.com/item/1242267" 
  $ time          : chr  "2016-01-02" "2016-01-02" "2016-01-02" "2016-01-02" ...
  $ changPrice    : num  0.997 1 1 1 1 ...
  $ classify      : Factor w/ 251 levels "","Clothing"....
  $ changesize    : num  -0.334 0 0 0 0 ...
  $ abs_changesize: num  0.334 0 0 0 0 ...
  
  library(dplyr)
  by_url &lt;- group_by(pricechange_0.5_2,url)
  url_datad &lt;- summarise(by_url,url_sd_chasize=(changesize -
                                                mean(changesize))/sd(changesize))
  </code></pre>
  
  <p>Here is a sample of my data.</p>
  
  <pre><code>A tibble: 10 × 3                                                                           
                               url  hangesize abs_changesize                                                                                      
                             &lt;chr&gt;      &lt;dbl&gt;          &lt;dbl&gt;                                                       
  http://item.yhd.com/item/1242267 -0.3343999      0.3343999
    http://item.jd.com/418657.html  0.0000000      0.0000000
  ...
  </code></pre>
  
  <p>Any other way to standadrize the <code>changesize</code> as <code>url</code>? </p>