title: '<' not supported between instances of 'float' and 'str' Error for Tukey HSD Test
body: <p>I get a strange error when running the Tukey test. I hope somebody is able to help me with this as I tried a lot. This is my dataframe:</p>
  
  <pre><code>    Name      Score
  1   A         2.29
  2   B         2.19  
  </code></pre>
  
  <p>This is my Tukey Test code:</p>
  
  <pre><code>#TUKEY HSD TEST
  
  tukey = pairwise_tukeyhsd(endog=df['Score'].astype('float'),     
                            groups=df['Name'],                     
                            alpha=0.05)                          
  
  tukey.plot_simultaneous()                                      
  plt.vlines(x=49.57,ymin=-0.5,ymax=4.5, color="red")
  
  tukey.summary() 
  </code></pre>
  
  <p>This is the error:</p>
  
  <pre><code>&lt;ipython-input-12-3e12e78a002f&gt; in &lt;module&gt;()
        2 tukey = pairwise_tukeyhsd(endog=df['Score'].astype('float'),     
        3                           groups=df['Name'],
  ----&gt; 4                           alpha=0.05)                          
        5 
        6 tukey.plot_simultaneous()
  
  /usr/local/lib/python3.6/dist-packages/statsmodels/stats/multicomp.py in pairwise_tukeyhsd(endog, groups, alpha)
       36     '''
       37 
  ---&gt; 38     return MultiComparison(endog, groups).tukeyhsd(alpha=alpha)
  
  /usr/local/lib/python3.6/dist-packages/statsmodels/sandbox/stats/multicomp.py in __init__(self, data, groups, group_order)
      794         if group_order is None:
      795             self.groupsunique, self.groupintlab = np.unique(groups,
  --&gt; 796                                                             return_inverse=True)
      797         else:
      798             #check if group_order has any names not in groups
  
  /usr/local/lib/python3.6/dist-packages/numpy/lib/arraysetops.py in unique(ar, return_index, return_inverse, return_counts, axis)
      221     ar = np.asanyarray(ar)
      222     if axis is None:
  --&gt; 223         return _unique1d(ar, return_index, return_inverse, return_counts)
      224     if not (-ar.ndim &lt;= axis &lt; ar.ndim):
      225         raise ValueError('Invalid axis kwarg specified for unique')
  
  /usr/local/lib/python3.6/dist-packages/numpy/lib/arraysetops.py in _unique1d(ar, return_index, return_inverse, return_counts)
      278 
      279     if optional_indices:
  --&gt; 280         perm = ar.argsort(kind='mergesort' if return_index else 'quicksort')
      281         aux = ar[perm]
      282     else:
  
  **TypeError: '&lt;' not supported between instances of 'float' and 'str'**
  </code></pre>
  
  <p>How can this error be resolved? Thanks in advance!</p>