Error in Measurements: Confused?

[gladius999]

This has been very comfusing. Is the error in taking a reading the smallest or half the smallest division? Say a standard 30cm ruler, you can judge to nearest 0.5mm but since you take 2 readings you need to double the error. But then in titrations using a lab burette you count the titre as half the smallest division? I am very confused here as I cannot decide what to put when a question in an exam asks for the error in the reading.

 

[Bob S]

For measurements of this type, where all the random uncertainties (not errors!) are equal and there are no systemantic uncertainties:
For n measurements:
Add all the measurements and divide by n to get average (don't use rms here).
Divide the uncertainty of one measurement by sqrt(n) to get the uncertainty of the average.
Bob S