The sample standard deviation \(\sigma\) was calculated according to (\ref{sd}) where \(M\) is the mean number of cells per square, \(N\) is the number of squares sampled, \(n_i\) is the number of cells in square nr. i, \(d_i^2\) is the square deviation of the number of cells in square i, and \(\sigma^2\) is the sample variance.