Read the instrument, not what you hoped for
- Meniscus — read the bottom of the curve, at eye level.
- Parallax — look straight on. Reading a scale from an angle shifts it.
- Zero the balance with the container on it (tare) before adding anything.
- Estimate one digit beyond the smallest marking. A ruler marked in millimetres gives you tenths of a millimetre, estimated.
Accuracy and precision are different
| Precise | Not precise | |
|---|---|---|
| Accurate | Readings cluster on the true value | Scattered around the true value |
| Not accurate | Cluster tightly in the wrong place | Scattered and wrong |
The dangerous quadrant is precise-but-not-accurate: consistent readings that are consistently wrong. A balance that was never zeroed produces beautiful, worthless data.
Which kind of error?
Systematic errors push every reading the same way — fix the instrument. Random errors scatter both ways — take more readings and average. A thermometer reading 2 °C high is which one?