Observational error, identical measurement results

[player1_1_1]

Homework Statement

hello, I have ten results of measurements, all are identical, and how i can count error? it is this: $$L = ( 20 \pm 0,1 ) cm$$ and all errors, mean squared error etc are 0, how can I count this now?

 

[player1_1_1]

up,,

 

[Redbelly98]

If the measurements came up all equal, then you could use whatever is the accuracy of your ruler. I've known people to use anywhere between 1/10 of a scale divisions to a full scale division, and metric rulers often use 1mm for the scale divisions.