title: Having trouble calculating mean squared error in sklearn python
body: <p>I am trying to fit a decision tree regressor to a dataset, and it is working but when I test it out by calculating mean squared error. I get an error that looks like this:</p>
  <pre><code>msee = mse(x_test, y_test)
  
  ValueError                                Traceback (most recent call last)
  ~\AppData\Local\Temp/ipykernel_17480/3348210221.py in &lt;module&gt;
  ----&gt; 1 msee = mse(x_test, y_test)
  
      ~\anaconda3\lib\site-packages\sklearn\metrics\_regression.py in mean_squared_error(y_true, y_pred, sample_weight, multioutput, squared)
          436     0.825...
          437     &quot;&quot;&quot;
      --&gt; 438     y_type, y_true, y_pred, multioutput = _check_reg_targets(
          439         y_true, y_pred, multioutput
          440     )
      
      ~\anaconda3\lib\site-packages\sklearn\metrics\_regression.py in _check_reg_targets(y_true, y_pred, multioutput, dtype)
          103 
          104     if y_true.shape[1] != y_pred.shape[1]:
      --&gt; 105         raise ValueError(
          106             &quot;y_true and y_pred have different number of output ({0}!={1})&quot;.format(
          107                 y_true.shape[1], y_pred.shape[1]
      
      ValueError: y_true and y_pred have different number of output (4!=1)
  </code></pre>
  <p>Here is the model code and a head of the df I am training the model on:</p>
  <pre><code>x = np.array(bat[[&quot;TB_x&quot;]])
      y = np.array(bat[[&quot;TB_y&quot;]])
      
      x_train, x_test, y_train, y_test = train_test_split(x, y, test_size= .2, random_state= 1)
      dt = DecisionTreeRegressor(max_depth= 10, random_state= 1, min_samples_leaf=.1)
      dt.fit(x_train.reshape(-1,1), y_train.reshape(-1,1))
      y_pred = dt.predict(x_test)
  
  
      index   Year    Age_x   AgeDif_x    Tm_x    Lg_x    Lev_x   G_x PA_x    AB_x    ... BA_y    OBP_y   SLG_y   OPS_y   TB_y    GDP_y   HBP_y   SH_y    SF_y    IBB_y
  0   19  2019    22.0    1.5 UCLA    P12 NCAA    38.0    72.0    58.0    ... 0.179   0.364   0.194   0.558   13.0    0.0 1.0 2.0 1.0 0.0
  2   24  2020    23.0    1.7 St. Leo SSC NCAA    20.0    86.0    69.0    ... 0.156   0.309   0.219   0.527   14.0    0.0 2.0 0.0 2.0 0.0
  6   45  2020    20.0    -0.8    Illinois    BTen    NCAA    13.0    58.0    47.0    ... 0.200   0.343   0.288   0.631   23.0    1.0 1.0 0.0 1.0 0.0
  7   46  2020    20.0    -0.8    Illinois    BTen    NCAA    13.0    58.0    47.0    ... 0.156   0.309   0.219   0.527   14.0    0.0 2.0 0.0 2.0 0.0
  8   49  2020    21.0    0.3 Miami (FL)  ACC NCAA    16.0    69.0    54.0    ... 0.200   0.343   0.288   0.631   23.0    1.0 1.0 0.0 1.0 0.0
  </code></pre>