% make a random vector of data just for example purposes
x = rand(25,1);
% define n for your nth element selection
n = 3 % for example every 3element
% make a vector with the nth elements
iSelect = 1:n:length(x);
% select every nth element
xSelect = x(iSelect)
You could also combine some of these lines of code to make it more compact
xSelect = x(1:3:length(x))
