How to find average nt,s,c from ntsc image?

clear;
close all;
clc;

 img = imread('C:\Users\US\Desktop\CompVision\images\lenna.jpg');
 %imresize(img,[200 200]);
 img = rgb2ntsc(img);
 figure('units','normalized','outerposition',[0 0 1 1]),
 subplot(221), imshow(img), title('Lenna Image');

 [r, c, ch] = size(img);
 img_nt =img(:,:,1);
 subplot(222), imshow(img_nt), title('Channel nt');
 img_s =img(:,:,2);
 subplot(223), imshow(img_s), title('Channel S');

 img_c =img(:,:,3);
 subplot(224), imshow(img_c), title('Channel C');