procedure ...
var i, T: integer;
m: real;
sumN: real; sigma:real;
begin
    m:=0;
    sumN:=0;
    T:= ... ;

    for i:=1 to T do
    begin
        sumN:= sumN + sqr(strToFloat(StringGrid1.Cells[1,i]) - m);
        m:=strToFloat(StringGrid1.Cells[1,i]);
    end;
    sigma:= sqrt(sumN/(T-1));
end;