Least count = smallest measurement a device can make. It defines the precision.
| Instrument | Typical Least Count |
|---|---|
| Metre scale | 1 mm = 0.1 cm |
| Vernier caliper | 0.01 cm = 0.1 mm |
| Screw gauge | 0.001 cm = 0.01 mm |
| Spherometer | 0.001 cm |
Accuracy = closeness to true value (affected by systematic errors) Precision = closeness of repeated measurements to each other (affected by random errors and least count)
A measurement can be precise but not accurate (e.g., consistently wrong due to zero error), or accurate but not precise (scattered around the true value).