Errors

Inaccuracies due to mistakes, poor technique or random variation. These are common but can be improved by taking many repeats. Data with small random error is precise. 

Innaccurate measurements in one drection only. Caused by poor calibration or poor technique. These cannot be improved by repeating. Data with small systematic error is reliable.

A particular type of systematic error where the instrument does not return to zero.

When the observer chooses some results and ignores others to suppoort a particular view.

A measurement that falls far outside the expected range and is therefore probably due to experimental error. Anomolie should be rejected because they skew the mean. However, it is very difficult to distiguish between anomolies and normal biological variation.

Ensuring that a measuring instrument gives correct readings by fixing known points and then constructing a scale around it.