In a study of bluebirds12 birds were trapped and their masses and wing spans were measured.

The data collected are shown in this table.



Mass/g

Wing

span/cm

9.1

20.0

9.9

18.0

12.1

17.5

13.2

17.0

10.8

20.5

10.2

18.5

12.3

19.0

11.9

19.0

9.3

20.0

10.4

17.5

11.7

18.0

12.4

19.5

Calculate the estimated standard deviation for the wingspan of the bluebirds, showing your answer to 3 significant figures.