Uncertainty - Origins
All measurements and readings have an associated uncertainty, which means that every time we record a data point, uncertainty is added to our final result.
This is not the only place uncertainty arises from though. In any experiment, we have to make assumptions to make our job easier. We assume the Earth is a perfect sphere, that a year is 365 days long, and that all beans in a jar are the same shape. In real life, footballs aren't perfect spheres, lines aren't perfectly straight, and beans don't all weigh the same! This slight difference in our mathematical models and reality adds uncertainty to our methods.
It is important for us as scientists to acknowledge the sources of uncertainty in our experiments so that we can try and keep them as small as possible and if necessary refine our experiments so that we can have more confidence in our results.