Change subscript of X_{ik} to X_{ki} to make it consistent with lhs.

[irudnyts]

In formulas the term X_{ik} is changed to X_{ki}.

Example:

Y_i = \beta_1 X_{1i} + \beta_2 X_{2i} + \ldots + \beta_{p} X_{pi} + \epsilon_{i} = \sum_{k=1}^p X_{ik} \beta_j + \epsilon_{i}

TO

Y_i = \beta_1 X_{1i} + \beta_2 X_{2i} + \ldots + \beta_{p} X_{pi} + \epsilon_{i} = \sum_{k=1}^p X_{ki} \beta_j + \epsilon_{i}