Based on ISO/IEC Guide 98-3 (GUM) principles
Every measurement we make in the laboratory has some degree of uncertainty. Think of it like taking a photo - even with the best camera, there's always some blur or imperfection. Measurement uncertainty helps us understand and quantify these imperfections.
This comes from repeated measurements of the same thing. Like weighing the same sample multiple times - you'll notice small differences each time. The calculator uses statistical methods to determine how much these variations affect your final result.
These are uncertainties we know about from other sources, such as:
A laboratory analyzing the copper content of a water sample.
5.82 5.76 5.85 5.79 5.81 5.78 5.83 5.80 5.77 5.84
Calibration of a 1000 µL pipette using gravimetric method.
998.2 1001.5 999.8 1000.3 997.9 1001.1 999.4 998.7 1000.6 999.2
pH measurement of an environmental water sample.
7.41 7.39 7.42 7.40 7.38 7.41 7.40 7.39 7.42 7.40
When using these examples with k=2 (95% confidence level), you should expect: