Error : Some sources of error are connected with the use of instruments, and can be summarized as:
(i) Instrumental error, arising from the accuracy of the measuring instrument itself.
(ii) Observational or reading error, arising from the precision with which that instrument may be read.
(iii) Setting or adjustment error, arising from the accuracy to which it is possible to set the instrument. It is also partly dependent on the skill and experience of the operator.
When writing up an experiment one usually estimates the maximum expected error. Sometimes worst-case errors are known from specifications on a piece of measuring equipment: sometimes the errors have to estimate as they cannot be known absolutely, e.g., misreading a dial where the pointer lies between two marks on a scale. In this last case, percentage error cannot be known absolutely. .
Percentage error can be obtained as:
Estimated error x 100 = Actual measurement