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Errors may be defined as the difference between a measured value and its true value. True value of a measurement is determined by taking the mean value of a series of repeated measurements. 
Types of Errors : Errors are classified in two types :-
  1. Systemic (Determinate) errors 
  2. Random (Indeterminate) errors

A. DETERMINATE ERRORS : Errors which can be avoided or whose magnitude can be determined is called as systemic errors. It can be determinable and presumably can be either avoided or corrected.

Systemic errors further classified as : 

  1. Operational and personal error 
  2. Instrumental error 
  3. Errors of method 
  4. Additive or proportional error 

1. Operational and personal error : 

Errors for which the individual analyst is responsible and are not connected with the method or procedure is called as personal errors. We can assign indeterminate errors to several sources, including collecting samples, manipulating samples during the analysis, and making measurements. e.g. Unable to judge colour change when errors occur during operation is called as operational error. 

E.g.- Transfers of solution, effervescence, incomplete drying, underweighting of precipitates, overweighing of precipitates, and insufficient cooling of precipitates. These errors are physical in nature and occur when sound analytical techniques is not followed.

2. Instrumental and Reagent errors :-

Errors occur due to faulty instrument or reagent containing impurities. E.g. - un-calibrated weights, un-calibrated burette, pipette and measuring flasks. 

3. Errors of Method :- 

When errors occur due to method, it is difficult to correct. In gravimetric analysis, error occurs due to Insolubility of precipitates, co-precipitates, post-precipitates, decomposition, and volatilization. 

In titrimetric analysis errors occur due to failure of reaction, side reaction, reaction of substance other than the constituent being determined, difference between observed end point and the stoichiometric equivalence point of a reaction. 

4. Additive or proportional errors :- 

Additive error does not depend on constituent present in the determination e.g. loss in weight of a crucible in which a precipitate is ignited. 

Proportional error depends on the amount of the constituent e.g. impurities in standard compound. 

B. INDETERMINATE ERRORS : These errors are also called accidental errors. Indeterminate errors arise from uncertainties in a measurement that are unknown and which cannot be controlled by the experimental list.

E.g. - When Pipetting out a liquid, the speed of draining, angle of holding the pipette, the portion at which the pipette is held, etc., would introduce indeterminate error in the volume of the liquid pipette out. 

Salient Features of Indeterminate Errors : The various salient features of indeterminate errors are :- 

  1. Repeated measurement of the same variable several times and subsequent refinement to the extent where it is simply a coincidence if the corresponding replicates eventually agree to the last digit.
  2. Both unpredictable and imperceptible factors are unavoidably incorporated in the results what generally appear to be ‘random fluctuations’ in the measured quantity.
  3. Recognition of specific definite variables which are beyond anyone’s control lying very close to the performance limit of an instrument, such as : temperature variations, noise as well as drift from an electronic circuit, and vibrations caused to a building by heavy vehicular-traffic.
  4. A variation that may be regarded as random by a slipshod analyst may at the same time prove to be quite evident and manageable by a careful observer, and 
  5. The average of a number of fine observations having random scatter is definitely more accurate, precise and, hence, more cogent than coarse data that appear to agree perfectly.