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Think about performing an experiment where you have to measure volumes in a graduated cylinder. If we had to measure 5mL several times, then precision would be having all of your measurements around the same volume. Perhaps all your measurements were around 5.3mL. It's not accurate because the measurement wasn't 5mL, but it is precise since all the measurements are close together. Accuracy would be measuring 5mL, 5.1mL, and 4.9mL. It's not precise since the measurements aren't too similar, but it's accurate since they are close to 5mL.
This is similar to what was said in class. Imagine you are running three trials of an experiment that require you to measure out a certain number of grams of an element. The balance that you use could give measurements numerically close to each other all three times. If this is the case, then the balance is precise. However, just because this balance gives precise measurements, does not necessarily mean that the measurements are accurate. The balance could be zeroed out inaccurately and the actual weight of the sample could be heavier or lighter than what the balance reads. However, if the balance is precise, all three measurements of the sample would be off by the same number.
Another example is if you are measuring the temperature of a substance and consistently get 39 degrees Celsius during 5 separate trials, but the substance is actually 35 degrees Celsius. This would mean that the measurements are precise, since you repeatedly get the same result, but they are not accurate because they are not the correct value.
Another example would be a titration lab. If you were to repeat the titration lab and continued to use about the same volume of the titrant in order to activate the indicator for each trial, then your lab would have a precise set of trials. However, your lab trials would only be accurate if the standardized answers (the actual pH values) are close to the actual pH value of the analyte.
If you were trying to pour exactly 5 mL of water in a beaker and the first three times you poured 5, 5.1, and 4.9, then your accuracy and precision is pretty high because it's close to 5 and the numbers are close together. Precision is related to repeatability so if the next three times you poured 5.6, 5.6, and 5.7, then although your measurements are not accurate they are precise because they are in the same range.
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