


<p>The influence measures defined by Barrett &amp; Ling (1992) are functions of two matrices \(\mathbf{H}_I\) and \(\mathbf{Q}_I\)
 defined as follows:</p><ul><li><p>For the full data set, the “hat matrix”, \(\mathbf{H}\), is given by
            \(\mathbf{H} = \mathbf{X} (\mathbf{X}^{T} \mathbf{X})^{-1} \mathbf{X}^{T} \),</p></li>
<li><p>\(\mathbf{H}_I\) is \(m \times m\) the submatrix of \(\mathbf{H}\) corresponding to the index set \(I\),
            \(\mathbf{H}_I = \mathbf{X} (\mathbf{X}_I^{T} \