title: Estimation to plot person-item map not feasible because items "have no 0-responses" in data matrix
body: <p>I am trying to create a person item map that organizes the questions from a dataset in order of difficulty. I am using the eRm package and the output should looks like follows:</p>
  <p>[person-item map] (<a href="https://hansjoerg.me/post/2018-04-23-rasch-in-r-tutorial_files/figure-html/unnamed-chunk-3-1.png" rel="nofollow noreferrer">https://hansjoerg.me/post/2018-04-23-rasch-in-r-tutorial_files/figure-html/unnamed-chunk-3-1.png</a>)</p>
  <p>So one of the previous steps, before running the function that outputs the map, I have to fit the data set to have a matrix which is the object that the plotting functions uses to create the actual map, but I am having an error when creating that matrix</p>
  <p>I have already tried to follow and review some documentation that might be useful if you want to have some extra-information:</p>
  <pre><code>    [Tutorial] https://hansjoerg.me/2018/04/23/rasch-in-r-tutorial/#plots
      [Ploting function] https://rdrr.io/rforge/eRm/man/plotPImap.html
      [Documentation] https://eeecon.uibk.ac.at/psychoco/2010/slides/Hatzinger.pdf
  </code></pre>
  <p>Now, this is the code that I am using. First, I install and load the respective libraries and the data:</p>
  <pre><code>    &gt; library(eRm)
      &gt; library(ltm)
      Loading required package: MASS
      Loading required package: msm
      Loading required package: polycor
      &gt; library(difR)
  </code></pre>
  <p>Then I fit the PCM and generate the object of class Rm and here is the error:</p>
  <blockquote>
  <p>*the PCM function here is specific for polytomous data, if I use a different one the output says that I am not using a dichotomous dataset</p>
  </blockquote>
  <pre><code>    &gt; res &lt;- PCM(my.data)
      &gt;Warning: 
      The following items have no 0-responses:
      AUT_10_04 AUN_07_01 AUN_07_02 AUN_09_01 AUN_10_01 AUT_11_01 AUT_17_01 
      AUT_20_03 CRE_05_02 CRE_07_04 CRE_10_01 CRE_16_02 EFEC_03_07 EFEC_05 
      EFEC_09_02 EFEC_16_03 EVA_02_01 EVA_07_01 EVA_12_02 EVA_15_06 FLX_04_01 
      ... [rest of items]
      &gt;Responses are shifted such that lowest
      category is 0.
      Warning: 
      The following items do not have responses on
      each category:
      EFEC_03_07 LC_07_03 LC_11_05
      Estimation may not be feasible. Please check
      data matrix
  </code></pre>
  <p>I must clarify that all the dataset has a range from 1 to 5. Is a Likert polytomous dataset</p>
  <p>Finally, I try to use the plot function and it does not have any output, the system just keep loading ad-infinitum with no answer</p>
  <pre><code>    &gt;plotPImap(res, sorted=TRUE)
  </code></pre>
  <p>I would like to add the description of that particular function and the arguments:</p>
  <pre><code>    &gt;PCM(X, W, se = TRUE, sum0 = TRUE, etaStart)
  </code></pre>
  <pre><code>    #X  
      Input data matrix or data frame with item responses (starting from 0); 
      rows represent individuals, columns represent items. Missing values are 
      inserted as NA.
  
      #W  
      Design matrix for the PCM. If omitted, the function will compute W 
      automatically.
  
      #se 
      If TRUE, the standard errors are computed.
  
      #sum0   
      If TRUE, the parameters are normed to sum-0 by specifying an appropriate 
      W. 
      If FALSE, the first parameter is restricted to 0.
  
      #etaStart   
      A vector of starting values for the eta parameters can be specified. If                         
      missing, the 0-vector is used.
  </code></pre>
  <p>I do not understand why is necessary to have a score beginning from 0, I think that that what the error is trying to say but I don't understand quite well that output.</p>
  <p>I highly appreciate any hint that you can provide me</p>
  <p>Feel free to ask for any information that could be useful to reach the solution to this issue</p>