The experiment should be conducted in a clear and systematic way to ensure the data is complete and of a high quality. In an experiment into the relationship between force, mass and acceleration a toy car of different masses runs down a ramp. The acceleration needs to be measured several times at each mass. The repeats would all need to be done in the same way and with care to ensure precise data. If you observe that a repeat is not similar to the others then it is good idea to repeat it.
It is also important to pay careful attention while the experiment is being carried out. It might be that the car starts to deviate from a straight line path; which if significant may mean that the method should be modified.
When using observations to collect data during an experiment it is important to be as accurate as possible. For example, when measuring the focal length of a converging lens using a distant object. The distant object is focused onto a screen. Different people might make the call as to whether the object has been sharply focused at slightly different positions, so it might be decided to use the same person to make all the observations.