Given a data matrix $mathbf X$ of $12 times 7$ size with samples in rows and variables in columns, I have calculated centered data $mathbf X_c$ by subtracting column means, and then computed covariance matrix as $frac{1}{N-1} mathbf X_c^top mathbf X_c$.
The dimensions of this covariance matrix are $7 times 7$. After that I have calculated the eigenvalue decomposition, obtaining eigenvector matrix $mathbf V$ with eigenvectors in columns.
Now I want to know about projections (i.e. principal component scores in PCA): is it $mathbf{V}^top mathbf{X}_c$ or $mathbf{X}_c mathbf{V}$?
mean_matrix = X - repmat(mean(X),size(X,1),1); covariance = (transpose(mean_matrix) * mean_matrix)/(12-1); [V,D] = eig(covariance); [e,i] = sort(diag(D), 'descend'); V_sorted = V(:,i); Best Answer
The projection is given by $mathbf{X}_c mathbf{V}$; principal component scores are columns of this matrix.
Your first formula cannot be computed, as $mathbf{V}^top$ is of $7 times 7$ size and $mathbf{X}_c$ has $12$ rows; the dimensions do not match, and the matrix product is not defined. Instead, $mathbf{V}^top mathbf{X}_c^top$ would be another correct formula, but it is simply a transpose of the first equation: $mathbf{V}^top mathbf{X}^top_c = (mathbf{X}_cmathbf{V})^top$. Here PC scores are in rows.
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