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"Should I do another trial?"
The number of trials needed to accurately determine any property is never defined. The number of trials is dependent on the precision and accuracy of the equipment, the desired accuracy, and the ability of the scientist.
Before deciding what is acceptable, the scientist should consider the precision and accuracy of the equipment and the desired accuracy. The precision and accuracy of the equipment can be determined experimentally, but most glassware found in general chemistry laboratories is labelled such that precision and accuracy can be noted by observing the graduations and markings on the glassware.
The desired accuracy should be determined on the basis of how the data or result will be used. If you are reporting the volume of your swimming pool, it might be acceptable to say "1500 gallons" when the actual volume is 1496.8 gallons since you were going to let the pool fill overnight. However, when working in a laboratory it is best to make the most accurate measurements possible during the course of an experiment. Thus, the result of any experiment is as accurate as possible.
The ability of the scientist is deemphasized in an experiment when the data is reproducible. Reproducibility is assessed by calculating the standard deviation of the data and comparing the standard deviation to the precision and accuracy of the equipment. If the experimental design is valid, when the standard deviation is minimized the ability of the scientist does not play a significant role in the acceptability of the data.
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