public int[][] stdDevColumns() {
        
        long[] sum = new long[columns];
        int[] count = new int[columns];
        int[] avg = averageColumns();
        
        for (int j = 0; j < columns; j++) {
            sum[j]=0;
            count[j]=0;
        }
        
        for (int i = 0; i < row_count; i++) {
            for (int j = 0; j < columns; j++) {
                if (data[j][i] != notValidNumber) {
                    sum[j] = sum[j] + (data[j][i] - avg[j]) * (data[j][i] - avg[j]);
                    count[j]++;
                }
            }
        }
        
        int[][] std = new int[2][columns];
        
        for (int j = 0; j < columns; j++) {
            if (count[j] > 0) std[0][j] = (int) Math.sqrt(sum[j]/count[j]);
            else std[0][j] = 0;
            std[1][j] = avg[j];
        }
        
        return std;
    }