The result of the who command simply gives you variable names as a cell array of strings ... it doesn't give you the variables themselves. Thus, DATA{i,1} is a string that is the name of a variable ... it is not the variable itself so you can't use .H syntax to get at the H field. You will probably need to use something like eval to get at the variable. E.g.,
scatter(eval([DATA{i,1} '.H']),eval([ORIGINAL{i,1} '.H']))
lm(i) = polyval(eval(DATA{i,1} '.H']),eval([ORIGINAL{i,1} '.H']));
This is admittedly ugly, but that is typically what you end up with when you have a multitude of variables in your workspace with similar names that you need to loop over. A better practice is often to have such variables gathered up into a single cell array instead. This makes accessing them in a loop much easier, and allows the use of MATLAB functions that operate on cell arrays (often avoiding loops altogether). E.g.,
