Sampling Errors:

There are two types of sampling errors:

(1). Sampling error.
(2). Non-sampling error.

The gap between the sample mean and population mean constitute the sampling error.

The gaps between various sample means are knows as sampling fluctuation.

Non-sampling error are two types:
(I) Systematic    (II) Unsystematic.

(I)  The errors arising out of the systematic tendency of the respondents to conceal  the fact and to over estimate or under-estimate values.    

(II) Errors arising in the process of collecting, recording, processing and analyzing the data due to carelessness or other mistakes are known as unsystematic errors.

In the case of census there is no sampling error but there are non sampling error.
In the case sampling there are both sampling error and non-sampling error.

By using a scientific sampling technique one can minimize the sampling error and as qualified investigators are included, the non-sampling error committed in the case of sample survey is also minimum.

The amount of non-sampling error in the case of census is much higher than the total amount of sampling and non-sampling error committed in the case of a sample survey  as less qualified investigator are involve in the case of census and the supervision, monitoring and quality control mechanism in the case of census.