How to estimate the total error?

I have a list of (systematic) errors obtained by calculating the percentage error of two sets of values. now from that list i don't know how to say what my total error was. these are the values:

-5.04

-3.09

-2.63

-0.20

0.00

1.40

3.88

I don't think that saying that the error was -5.04 to 3.88 is correct. I think I have to calculate the standard deviation or the standard error but I am not sure which. I know the formula for the standard deviation but for the standard error i'm not sure if it is the square root of the sum of the squares. What do you suggest I do to obtain the total error?