Population forecasting of small areas or ethnic groups Stockholm, 21 st November 2008 Ludi Simpson University of Manchester

Population forecasting: 2 practical dilemmas Theory without software –Cohort component framework –Multi-regional, probabilistic, socially disaggregated –Academic research –Project-specific Priority problems without stable data –Sub-national and non-standard areas –Ethnic or national groups –Non-cohort, non-component methods un-informative

Version 1 (1999) – version 3 (2005) Local/Regional government concerns –Replicate and develop national agency work –Population, housing, and labour force –Impact of local development policies –Ethnically diverse populations –Large populations and systems of small populations Standard national methods –applied to 1 or more ‘groups’, named by the user

POPGROUP design Principles and practice Excel input files Excel output files Macros do work of structuring files, validating data, projections and most interrogation Easy start, then develop – the future is not what it used to be Integrate estimates and forecasts

Each component of population change is one input file P t+1 = P t + B – D + I UK – O UK + I OV – O OV Seven input files, plus: Constraints : population, housing and employment Special populations (students, armed forces) Dwellings (vacancy, second homes, sharing households) Jobs (commuting, unemployment) Natural change MigrationBase popul- ation Future popul- ation

Each input file represents a collection of assumptions for one component

Mathematical approach to combine the varied availability of demographic rates, past counts, and targets Initial population and one age- schedule each for rates of fertility, mortality, migration The only required inputs Differentials for groups, years, age-sex groups Used to create initial projections for year y+1 Counts of births, deaths, migration flows in period Used to constrain the initial projection Population, housing or labour force Used to further constrain the migration flows

Projecting small populations (areas or ethnic groups) Few data for non-standard areas Time series are unstable –Recent past may not indicate an underlying level of fertility, mortality, migration Alternatives provide neither age nor components –Mathematical extrapolation –Shift share –Housing units

Example 1: replicating District population and household projections. Detailed data available

Household outputs

Example 2: Wards of Bradford. Births, deaths and population available for recent years Each area’s counts of births indicate its past level of fertility. Trends that are expected to affect all areas are entered on the ‘All-groups’ sheet Expressed as a differential to a standard schedule, the area level is continued to the future Local trends may be identified and used in assumptions for the future

The impact of fertility file options on the Total Fertility Rate 4. Unusual trends may be continued in future Forecasts Training phase Total Fertility Rate 1. Each area’s counts of births 2. Age-trend on All-areas sheet, from GAD projections 3. Each area’s differential, maintained in future

The impact of migration file and constraint options on migration

A cohort component projection allows understanding of population dynamics from incomplete information Birmingham city: eight ethnic groups –Estimate components of change for each group –Estimate population change due to each component –Present the decomposition of expected population change

Example 3: The impact of housing plans on population Population and housing change, a two-way relationship Regional planners are interested –Large areas: how much land should be released for house-building? –Small areas: what is the impact on services of planned house-building? Greater numbers of houses built may result in… –More in-migration –Less out-migration –Lower household size –Higher proportion of vacancies, or of holiday-homes

POPGROUP alters migration to meet housing constraints (running HOUSEGROUP in the background) What’s the impact on population of planned housing developments? What’s the change in number of households and dwellings each year?

National Parks: ageing populations Decomposition of household change Population size Population age structureHeadship Total change Peak District Cairngorms

Peak District National Park: new housing will not create a workforce (unless the migration age structure changes) Population change – Dwelling led projections ( ) Scenario % Population change% Working age pop change Recent migration continues -14%-35% 48 dwellings p/a-6.3%-29% 95 dwellings p/a1.1%-22% 150 dwellings p/a9.9%-13%

Milton Keynes spill-over to Aylesbury Vale (2 wards selected) KEY Green: migration as estimated for (‘Trend- based’) Red: Zero new dwellings. Black: alternative planned house- building

Discussion Good forecasts incorporate good estimates Detailed small area forecasts are possible with few data, if they are relevant data