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Census of Population and Housing: Census dictionary
Reference period

Improvised home

These include sheds, tents, humpies and other improvised dwellings, occupied on Census Night. This category also includes people sleeping out, such as those sleeping on the streets, in abandoned buildings, under bridges or in cars.  

See also Homelessness enumeration and Dwelling structure (STRD).


Imputation is a statistical process for predicting values where no response was provided to a question and a response could not be derived. In the Census, we impute key demographic variables (sex, age, marital status, and usual residence) where no Census form is returned or when a respondent does not respond to one of these questions.

Where a private dwelling is identified as occupied on Census Night but a Census form has not been returned, people are imputed into that dwelling - both the number of people and their key demographic characteristics.

If a person in a non-private dwelling did not return a form, their demographic characteristics are imputed from people in similar non-private dwellings.

Where a person responds to the Census but does not answer the age, sex, marital status or usual residence questions, values are imputed using other information on the form as well as the distribution of these data items in the responding population. 

Imputation flag variables enable users of Census data to quantify the number of imputed records, for applicable data items, in a given population.

For more information on imputation see the 2021 Census methodology.

See also Derivation, Non-response and Hotdecking.

Intelligent Character Recognition (ICR)

2021 Census data was processed using Intelligent Character Recognition (ICR) technology for the paper forms. Specialised computer software is used to interpret the handwriting on images taken of each page of the Census form. Once recognised, answers to Census questions were coded to the appropriate category of the relevant classification, for example Religion, Occupation, etc.

See also Data processing.

Internal migration

Internal migration is the movement of people across a specified boundary within Australia for the purpose of changing their place of usual residence. Information on internal migration within Australia is available from the Census.

The Census asks a series of questions relating to each person's usual address which can be used to identify the change of address of people for one year prior to the Census data, and for five years prior to the Census date. Data collected in the Census only reflect movements which coincide with these particular points in time in the intercensal period (i.e. one year ago and five years ago), even though there may have been multiple movements during this period.

Household mobility indicators are also derived using this information. Note that persons temporarily absent, visitors, and households containing only visitors, are excluded from these variables. 

See also Household one year mobility indicator (MV1D), Household five year mobility indicator (MV5D), Place of usual residence one year ago (PUR1P), Place of usual residence five years ago (PUR5P),  Usual address indicator Census night (UAICP), Usual address one year ago indicator (UAI1P), Usual address five years ago indicator (UAI5P), and Usual residence.

Interviewer household form

The Interviewer household form is used in Aboriginal and Torres Strait Islander communities (and areas where language differences or other factors make use of the standard self-enumeration forms impractical). The Interviewer household form is an interview based Census form which is used to record the details of up to 12 people in a household, and some dwelling data. If there are more than 12 people in a dwelling, additional Interviewer household forms are used.

To view a PDF version of the Interviewer household form, see Sample copies of the 2021 Census paper forms.

Introduced random error/perturbation

Under the Census and Statistics Act 1905 it is an offence to release any information collected under the Act that is likely to enable identification of any particular individual or organisation. To minimise the risk of identifying individuals in aggregate statistics, a technique has been developed to randomly adjust values. Random adjustment of the data, known as random error or perturbation, is considered to be the most satisfactory technique for avoiding the release of identifiable data while maximising the range of information that can be released.

See also Confidentiality.