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Derivations and imputations

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Derivation is the process where some variables are assigned values based on responses to other questions, or (where no response has been provided) from other family members present in the same dwelling.

Variables that may be derived from responses given by other family members present in the same dwelling are:

• Country of Birth of Person (BPLP)
• Birthplace of Male Parent (BPMP)
• Birthplace of Female Parent (BPFP)
• Language Spoken at Home (LANP)

If there is insufficient information provided to derive a response for these items, they are determined to be 'Not stated'.

In addition, the derivation process is used to create new variables by combining responses from a number of questions. Variables which are created this way include:

• Mortgage Repayments (monthly) Dollar Values (MRED)
• Rent (weekly) Dollar Values (RNTD)
• Tenure Type (TEND)
• Labour Force Status (LFSP)
• Core Activity Need for Assistance (ASSNP)

Imputation is a statistical process for predicting values where no response was provided to a question and a response could not be derived.

Where no Census form is returned, the number of males and females in 'non-contact' private dwellings will be imputed. In addition, the following key demographic variables may also be imputed, if they are 'Not stated':

• Age (AGEP)
• Place of Usual Residence (PURP)
• Registered Marital Status (MSTP)

The primary imputation method used for the 2011 Census is known as 'hotdecking', Other imputation processes use probability methods. In general the hotdecking method involves locating a donor record and copying the relevant responses to the record requiring imputation. The donor record will have similar characteristics and must also have the required variable(s) stated. In addition the donor record will be located geographically as close as possible to the location of the record to be imputed. The match must occur within the same Capital City or Balance of State.

The methodology for imputation is tailored to two situations. Firstly, where no Census form has been returned and secondly where a partially completed form was returned.

No Census form returned - private dwelling: Where a Census collector has identified that a private dwelling was occupied on Census Night but a Census form was not returned, the number of males and females normally in the dwelling and their key demographic variables may require imputation. In these cases, the non-demographic variables are set to 'Not stated' or 'Not applicable'.

The 'No Census form returned' scenario has two variations. Firstly, where no form was returned but the collector was able to ascertain the number of males and females from a resident of the dwelling, or in a small number of cases a building manager or neighbour. And secondly, where no form was returned and the number of males and females remains unknown.

For records where the number of males and females is unknown, two imputation processes are required. Initially these records must have their number of males and females imputed using hotdecking. Then a second imputation (also using hotdecking) is run to impute the key demographic variables.

To hotdeck the number of males and females, the donor records must meet several conditions:

• They must be records where no Census form was returned but where the number of males and females was ascertained by the collector;
• They must have a similar Dwelling Structure (STRD) to the record to be imputed; and
• They must be located geographically as close as possible to the location of the record to be imputed.

The number of males and females are the only data copied from the donor record in the first hotdecking process.

In the next process, the records which have just had their number of males and females imputed, are subjected to the same hotdecking process as those records where the number of males and females had been ascertained.

This hotdecking process imputes the key demographic variables. Again the donor records must meet several conditions:

• They must be records where everyone within the dwelling provided all their demographic characteristics;
• They must have similar Dwelling Structure (STRD) and Dwelling Location (DLOD);
• They must have identical counts of males and females; and
• They must be located geographically as close as possible to the location of the record to be imputed.

The key demographic variables are then copied from the donor records to the records requiring imputation.

No Census form returned - non private dwelling: Where a person in a non-private dwelling did not return a form, their demographic characteristics are copied from another person in a similar non-private dwelling using Type of Non-Private Dwelling (NPDD).

Census form returned: Where a form was returned, some or all of the demographic characteristics may require imputation. If Registered Marital Status and/or Place of Usual Residence are 'Not stated' they are imputed using hotdecking, whereas Age is imputed based on distributions obtained from previous Censuses.

Registered Marital Status imputation is carried out by finding a similar person in a similar responding dwelling based on the variables:

• Sex (SEXP);
• Relationship in Household (RLHP);
• Age (AGEP);
• Dwelling Type (DWTD); and
• Type of Non-Private Dwelling (NPDD).

Registered Marital Status is only imputed for persons aged 15 years and over, and set to 'Not applicable' for persons aged under 15 years.

Where a complete usual address on Census Night is not provided, the information that is provided is used to impute an appropriate Mesh Block (as well as Statistical Area Level 1 and Statistical Area Level 2). A similar person in a similar dwelling is located, and missing usual residence fields are copied to the imputed variable.

These are based on the variables:

• Residential Status in a Non-Private Dwelling (RLNP);
• Dwelling Location (DLOD); and
• Type of Non-Private Dwelling (NPDD).

Where date of birth or age details are incomplete or missing, the variable Age (AGEP) is imputed based on distributions for particular populations (for example, male or female; marital status, Indigenous or Non-Indigenous and state/territory of usual residence). Factors affecting age imputation include any reported labour force activity, educational institution attending and other family member relationships and ages.

For 2011, dwelling address derivation and imputation is being introduced. Private dwellings that have an incomplete or no address will have a mesh block code derived from adjacent dwellings listed in the collector record book. If a dwelling mesh block code is unable to be derived the dwelling will be imputed into a Mesh Block located within the relevant collector workload. Imputation of the Mesh Block code is a probability proportionate method based on distributions of coded dwellings across mesh blocks contained within a collector workload. These distributions are based on Dwelling Structure (STRD).

For previous censuses the output geographic boundary (Collection District (CD)) was the same as the collection geographic boundary and therefore a dwelling address was automatically coded to the code of the collector's district.

Records that have required imputation can be identified using the Imputation flags:

• Imputation Flag for Age (IFAGEP);
• Imputation Flag for Number of Males and Females in Dwelling (IFNMFD);
• Imputation Flag for Place of Usual Residence (IFPURP); and
• Imputation Flag for Registered Marital Status (IFMSTP).

See also Imputation variables.

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