Derived units are those expressed in terms of fundamental units. Primary or fundamental units cannot be expressed in terms of other units.

Error of a Quantity which is a constant multiple of another quantity will be a constant multiple of error of first one.

Systematic errors are caused by poor calibration of instruments.

The possible way of eliminating systematic error is the replacement of instruments. Systematic errors are caused by poor instrument calibration.

Secondary standards are fixed and used in industrial laboratories. Working standards as its name suggests is used for day to day measurements. International standards are accepted internationally and primary standards are used in different parts of world which will not be accessible outside for calibration.

Random errors will remain in a system even if we remove all systematic errors. Random errors are also known as residual errors.

Displacement between two positions represented as x±Δx and y±Δy is Iy-xI and error in displacement is (Δx2 +Δy2)1/2.

Zero error refers to a false indication of an instrument when the true value is zero and zero error can be treated as an instrumental error.

SI unit of Luminous intensity is candela represented by cd. Luminous intensity is a Photometric quantity.

Gross errors are mostly due to lack of knowledge, judgment and care on the part of the experiment. That is Gross error is caused by careless handling.