Step 1: Understand the concept of rank and determinant.
The rank of a matrix is the maximum number of linearly independent rows (or columns). If a \(3 \times 3\) matrix has rank less than 3, its rows (or columns) are linearly dependent.
Step 2: Relation between rank and determinant.
If the rank of a square matrix is less than its order (here, less than 3), then its determinant is zero.
The percent Fe content of a random sample consisting of five observations is shown:

If the mean grade of the stockpile is estimated using the above data, the standard error of the mean grade, in %, is _______ (rounded off to 3 decimal places).