The Survey of Family, Income and Employment Dynamics (SoFIE) began in October 2002 and is the largest longitudinal survey ever run in New Zealand. The primary focus of SoFIE is to look at the changes in individual, family and household income, and the factors that influence these changes, such as involvement in the labour force, and family composition. The survey re-interviews the same group of individuals over eight years (or 'waves'), in order to build a picture of how their circumstances and lifestyles change over time.
The statistics in the attached tables present selected information collected during the first three waves of SoFIE, up to September 2005.
The target population for SoFIE is the usually resident population of New Zealand living in private dwellings. This means the survey excludes overseas visitors who intend to stay in New Zealand for less than 12 months, non-New Zealand diplomats and diplomatic staff and their dependents, members of non-New Zealand armed forces stationed in New Zealand and their dependents, and people living in institutions or in establishments such as boarding houses, hotels, motels and hostels.
For practical reasons, the population surveyed is restricted to people whose usual residence at the time of sample selection is a permanent private dwelling on the North Island, South Island or Waiheke Island.
At wave one, a total of about 15,000 randomly-selected households were approached to take part in SoFIE. Approximately 11,500 households agreed to be interviewed, and data was collected from over 22,000 eligible individuals aged 15 years and over. All adults responding at wave one, and children aged under 15 years for which data was collected in wave one, are known as original sample members (OSMs). The intention is to re-interview all OSMs aged 15 years and over in subsequent years, regardless of changes in their place of residence. OSM children will not be interviewed directly until they turn 15. From the second interview onwards, other members of an OSM's household who are not OSMs ('cohabitants') are also interviewed while they remain living with an OSM. Cohabitants are asked a reduced set of questions and are not followed up if they leave the OSM's household.
SoFIE is conducted using computer-assisted interviewing. Interviewers use laptop computers to administer the questionnaire face to face with the respondent in the respondent's home. There are two separate questionnaires used to collect information for SoFIE. The household questionnaire is answered by one person in each household and collects household characteristics. A personal questionnaire is completed with every OSM in the household aged 15 years and over. A slightly shorter version of the personal questionnaire is completed with any adult cohabiting with an OSM adult from wave two onwards. Children aged under 15 years are not interviewed; instead, a nominated parent/other adult is asked questions about them.
The household questionnaire contains two sets or 'modules' of questions:
- Standard of living.
The personal questionnaire contains eight standard modules:
- Child (if the respondent is a nominated adult answering about a child)
- Labour market history
- Labour market
The SoFIE questionnaires collect both point-in-time data and time-spell data. Point-in-time data relates to a single date, usually the interview date (eg the respondent’s educational qualifications as at the interview date). Time-spell data relates to a period of time with a defined start and end date reported by the respondent (eg the period of time a respondent lived with a family member, or the length of time a person worked for a particular employer). The analysis in this release includes uncompleted time spells, that is time spells that were ongoing at the time of the third interview.
Over the eight waves of the survey, different modules will be added to gather a more complete picture of the influences on individual and household circumstances. In waves two, four, six and eight a net worth module, collecting type and value of assets and liabilities will be included. In waves three, five and seven a health module, collecting self-rated health status, chronic diseases, major health events, risk factors and health service use will be included.
Wave one of SoFIE was conducted from 1 October 2002 to 30 September 2003, wave two from 1 October 2003 to 30 September 2004, and wave three from 1 October 2004 to 30 September 2005. The original sample was spread over the 12 months in the first wave so that interviewing was continuous over the year. The interview for each subsequent wave will always be in roughly the same month as the interview for wave one. At each interview, the respondent is asked to recall information about a specific annual reference period. The annual reference period is the 12 months prior to the month of interview. This means that SoFIE data published in this release relates to a three-year period for each respondent that falls somewhere between 1 October 2001 and 30 September 2005.
In a longitudinal survey the response rate for the survey will decline over time as individuals are unable to be located, leave the country, or die. Minimising attrition (loss of respondents) is very important, because of the cumulative effect of non-response over time. Statistics NZ is putting considerable effort into maintaining contact with respondents in order to be able to interview them in subsequent years, thus maintaining a high response rate for SoFIE.
Despite this, Statistics NZ has been unable to collect valid data from all selected eligible individuals. The most common reasons for this were that a respondent was unable to be contacted, or that a respondent was not able to provide all the relevant information asked for.
For wave one, approximately 77 percent of eligible households responded. In the second year of the survey, 87 percent of all respondents from wave one responded again. In the third year, 80 percent of all respondents from wave one responded.
A basic survey weight is attached to each record to indicate the probability of that unit being included in the sample. Two types of adjustment are then applied to the basic survey weights to improve the reliability of the survey estimates. The basic weights are first inflated to adjust for non-response, and are then further adjusted to ensure that estimates of relevant population characteristics match known population totals. The population totals used for SoFIE are derived from population estimates produced by Statistics NZ Population business unit for counts for different age/sex groups.
Note that estimation methodology has been revised and the population estimates updated since wave 2. This means that data in this release are not necessarily comparable with similar data produced for earlier SoFIE releases. It is expected that the data will be routinely updated at every wave.
Some respondents are unable to provide complete information. In these cases, missing values are imputed for all key fields. The key fields for SoFIE are age, ethnicity, income and pay details. Where possible, information is imputed deterministically, using other information reported by the respondent to provide a likely estimate for the missing value. When deterministic imputation is not possible, a 'hot deck' imputation method is used. This method involves selecting another respondent with similar characteristics to become the 'donor' and provide the imputed value.
Reliability of survey estimates
The initial SoFIE sample comprised approximately 11,500 responding private households and 22,000 adults sampled within them, on a statistically representative basis from rural and urban areas throughout New Zealand. Information is collected from each member (including children) of a sampled household that falls within the scope of the survey and meets survey coverage rules. In wave three there were just over 18,200 responding OSM adults.
Two types of error are possible in estimates based on a sample survey: sampling error and non-sampling error. Sampling error is a measure of the variability that occurs by chance because a sample rather than an entire population is surveyed. All sampling errors for SoFIE are measured at the 95 percent confidence interval. The estimates in the tables have had specific sampling errors calculated for them that are available on request.
Non-sampling errors include errors arising from biases in the patterns of response and non-response, inaccuracies in reporting by respondents, and errors in the recording and coding of data. Statistics NZ endeavours to minimise the impact of these errors through the application of best-practice survey methods and monitoring known indicators (eg non-response).
Due to rounding procedures, table totals may differ from the sum of individual cells. All counts and values in the tables have been rounded to the nearest hundred.
A full set of definitions is available from Statistics NZ. The definitions listed refer only to terms used in the release commentary.
Annual reference period
Annual data is that in which one value is collected for a 12-month period. The 12-month period is defined by the respondent's start-of-annual-period date and end-of-annual-period-date. An example of annual data is the amount of income the respondent received from interest over an annual reference period.
End of annual period date
This is the date which marks the end of the annual reference period for the respondent.
The definition of the family is based on the concept of a family nucleus. A family nucleus is a couple, with or without child(ren), or one parent and their child(ren) where the children do not have partners or children of their own living in the same household. Note that the children can be of any age.
Family type describes the type of family a person is living in, not who is in the family. Someone who is in a couple at the first interview of SoFIE, for example, may also be in a couple a year later – however they may have changed partners during the course of the year.
A household is either one person who usually resides alone, or two or more people who usually reside together and share facilities (such as eating, cooking, bathroom and toilet facilities, and a living area).
Household type describes the type of household a person is living in, not who is in the household.
Household enumeration date
The household enumeration date is the date when all eligible members of a household have been identified and recorded in the household questionnaire.
Labour force involvement
A respondent's labour force involvement is defined as being either employed, not employed but seeking work, or not employed and not seeking work. The definitions for involvement in the labour force are aligned with, but not identical to, the concepts and definitions used in the Household Labour Force Survey.
This definition includes all individuals in the working-age population who worked for one hour or more, either as an employee or in self-employment; or worked without pay for one hour or more in work which contributed directly to the operation of a farm, business or practice owned or operated by a relative; or had a job but were not at work due to own illness or injury, personal or family responsibilities, bad weather or mechanical breakdown, direct involvement in industrial dispute, or leave or holiday.
Not employed but seeking work
This definition includes all individuals in the working-age population who were without a paid job and seeking work. Note that this is not the same as the official measure of unemployment derived from the Household Labour Force Survey, because of the difficulty respondents may have in remembering details of their job search activity and availability to start a new job for dates up to a year earlier
Not employed and not seeking work
This definition includes any person in the working-age population who is neither employed nor seeking employment: for example, people who are retired, or have personal or family responsibilities; people attending educational institutions; people permanently unable to work due to disabilities; and people not actively seeking work.
People responding in all waves of the survey are called longitudinal respondents. Those people who have responded in waves 1, 2 and 3 of SoFIE are the current longitudinal responding individuals.
Employee earnings quintiles divide the population into five groups by ranking people by the amount of employee earnings they receive. The bottom quintile (quintile 1) is the lowest 20 percent of the population in terms of employee earnings, while the top quintile (quintile 5) is the highest-earning 20 percent of the population.
The quintile boundary is the dollar value at which the quintile falls. Given that the bottom quintile for employee earnings is earnings less than $274, the quintile boundary between quintiles 1 and 2 is $274.
A time spell is a period of time reported by a respondent. For example, the period of time a respondent is in paid employment is referred to as an employment time spell or 'spell'.
In a longitudinal survey, interviews are conducted with the same people repeatedly over time. SoFIE is thus made up of cycles, or 'waves', of interviewing. The wave length (ie the time between each wave) for SoFIE is one year, which means that respondents are interviewed annually. The first time respondents were visited was known as wave one, the second time as wave two, and so on.
Weekly employee earnings
These are the earnings reported by those people with at least one spell of employee earnings during the year. Weekly employee earnings include gross salary/wages, regular allowances, and commissions and bonuses received as usual pay from a spell of employment, divided by the duration of the spell (in weeks). Self-employment income is not included, and redundancy, holiday pay, back pay and other irregular payments are also excluded.
For more information, follow the link from the Technical notes of this release on the Statistics New Zealand website.
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Survey of Family, Income and Employment Dynamics (Wave 4): September 2006 will be released at a future date (to be advised).
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