PhD title:Population projections for small areas and ethnic groups - developing strategies for the estimation of demographic
rates
Lee WilliamsonCCSR
Cathie Marsh Centre for Census & Survey Research
The University of Manchester
Deriving age-specific fertility rates by ethnic group at the ward level for Bradford: an
assessment of 6 promising strategies
Background to research project
• The PhD is CASE partnered with Bradford MDC using data provided by Bradford MDC
• Core problem of creating demographic rates to be used in population projections where there is very little data available (small areas, ethnic groups or both)
• Different strategies of both methodological approach and data sources will be used to provide the estimates of the demographic rates
• The impact of the demographic rates will be assessed by implementing the rate estimates in projection software Popgroup provided by Bradford MDC.
• This presentation will focus only on fertility rates
Overview of Presentation
Overview:
– Introduction to the Bradford district
– Introduction to the 6 different sets of age-specific fertility rates (ASFRs)
– Assessing the 6 different sets of rates by using them in population projections and comparing projected births against actual births(1) ethnic group at the ward-level for 1991-1998 (2) at the ward-level only for 1991-2000
– Discussion of findings
Overall to compare the 6 sets of ASFRs in population projections in order to assess whether using detailed ethnic-specific ward-specific information produces the more accurate projected births
Bradford Metropolitan District in 1991
• Bradford is a multicultural district
• 30 wards
• Ward sizes rangefrom 13,000 to 23,000
• Total population 480,000 (1991)
The 6 different sets of age-specific fertility rates (ASFRs)
I. GAD England rates for 1991 (Government Actuary Dept. from the England national-level population projections)All other sets of ASFRs were created with using maternity records that record ethnic group, using an average of 5 years worth of births (1989-1993) for 1991 rates
II. Bradford district ratesIII. Ethnic group only rates at the district level (smoothed by the Hadwiger function due
to ragged ASFRs)IV. Ward only rates equal across all ethnic groups (smoothed by the Hadwiger function)
The final sets of ASFRs are based on a ‘grouping’ strategy which, overall, grouped wards together based on cluster analysis using fertility level (TFR) and 1991 Census variables which have been used in many deprivation measures and also considering the 1991 ONS classifications for wards. Thus, the ‘groupings’ ASFRs were based on more events (births) and larger populations of childbearing women
V. The ‘grouping’ rates specific for each ethnic group (smoothed by the Hadwiger function)
VI. The ‘combined best method’, where there were over 100 births recorded in the 1989-1993 period (average of 20 births) using that ward-specific ethnic group rate and, if not, alternatively using the ‘groupings’ rate (all smoothed by the Hadwiger function)
The ‘combined best method’ will either be the ward-specific ethnic group rate, or a ‘grouping’s’ ethnic-specific ward-level fertility rate.
1991 ASFR curves: GAD England and Bradford district
Fertility schedules for Bradford district and GAD England in 1991
0
0.02
0.04
0.06
0.08
0.1
0.12
15 20 25 30 35 40 45 50age
fert
ility
rate
Bradford 1991 (TFR 1.98)
GAD England 1991 (TFR 1.83)
1991 ASFR curves: ethnic group rate at the district-level
Fertility curves for all ethnic groups at the Bradford district-level All ethnic groups in Bradford
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
15 20 25 30 35 40 45age
rate
Bradford
White
Black
Indian
Pakistani
Bangladeshi
Other
1991 ASFR curves: ethnic group rate at the district-level smoothed by the Hadwiger
functionFertility curves for all ethnic groups at the Bradford district-level
All ethnic groups in Bradford
0.000
0.050
0.100
0.150
0.200
0.250
0.300
15 20 25 30 35 40 45
age
rate
WHITE
BLACK
INDIAN
PAKISTANI
BANGLADESHI
OTHER
1991 ASFR curves: ward-level rates smoothed by the Hadwiger function
low TFR and high ward TFRs compared with Bradford district (1.98)
all wards with TFR below 1.98
.00
.05
.10
.15
15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49age
rate
Baildon
Bingley
Bingley Rural
Bolton
Clayton
Craven
Eccleshill
Great Horton
Idle
Ilkley
Odsal
Queensbury
Rombalds
Shipley West
Thornton
Wibsey
Worth Valley
Wyke
all wards with TFR above 1.98
.00
.10
.20
15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49age
rate
Bowling
BradfordMoor
Heaton
KeighleyNorth
KeighleySouth
KeighleyWest
Little Horton
Shipley East
Toller
Tong
Undercliffe
University
1991 ASFRs: the ethnic-specific ‘groupings’ rates
19 final fertility groupings to ensure 20 or over births Groups or combined grouping
Women* TFR
White suburbia 1 24,200 1.57
White middling Britain 2 14,400 1.76
White established owner-occupier/prosperous areas 3 9,900 1.58
White lower status owner-occupier/industrial areas 4 32,200 1.77
White urban deprived industrial areas 5 13,600 1.99
White University ward 1,500 1.01
Black suburban more established areas+low status own-occ/industrial areas 1,2,3,4 700 1.66
Black urban deprived industrial areas 5 800 1.38
Indian suburban more established areas 1,2,3 600 1.57
Indian lower status owner-occupier/industrial 4 1,000 1.66
Indian urban deprived industrial areas 5 1,900 1.87
Pakistani suburban more est. areas 1,2,3 500 3.66
Pakistani lower status owner-occupier/industrial areas 4 3,200 4.87
Pakistani urban deprived industrial areas 5 7,800 3.87
Bangladeshi suburban more established areas+low status own-occ/ind areas 1,2,3,4 400 3.06
Bangladeshi urban deprived industrial .areas 5 400 3.85
Other suburban more established areas 1,2,3 300 3.15
Other lower status owner owner-occupier/industrial areas 4 300 3.05
Other urban deprived industrial areas 5 700 1.93*rounded to nearest hundred
1991 ASFR curves: the ethnic-specific ‘groupings’ rates smoothed by the Hadwiger
function
• 19 sets of fertility rates were smoothed using the Hadwiger curve
• Some sets of ASFRs are still very ragged
White IndianSuburbia Suburban more established areas
age of women
49
47
45
43
41
39
37
35
33
31
29
27
25
23
21
19
17
15
.3
.2
.1
0.0
i_subc_r rate
pred i_subc_r indian
suburban more estab
age of women
49
47
45
43
41
39
37
35
33
31
29
27
25
23
21
19
17
15
.14
.12
.10
.08
.06
.04
.02
0.00
w_suburb rate
pred w_subu_r white
suburbian rate
Comparing the 6 different sets of ASFRs using projected births against actual
births• The different set of age-specific fertility rates being tested for
accuracy are:
I. GAD England ratesII. Bradford district ratesIII. Ethnic group only rates (smoothed by the Hadwiger function)IV. Ward only rates (smoothed by the Hadwiger function)V. The ‘grouping’ rates specific for each ethnic group (smoothed by the Hadwiger function)VI. The ‘combined best method’ rates (smoothed by the Hadwiger function)
• The ‘combined best method’ will either be:ethnic-specific ward-specific rates if over 20 births occurring on average (1989-1993) or the ethnic-specific ‘grouping’ fertility rates
Error measure is the Mean Absolute Percentage Error (MAPE)
n
ttPE
nMAPE
1
1
100*
t
ttt Y
FYPE
mean absolute percentage error
the percentage error (PE)
(Makridakis 1998:44)
• The problem with using the MAPE is that there must be at least 1 observed birth (Yt) occurring, otherwise it cannot be used (cannot divide by zero).
• For example, for ethnic group Black there were 9 wards where there were births occurring, however, due to the small numbers of births occurring the MAPEs could only be calculated for 7 wards.
Comparing projected births to actual births for period 1991-1998
using the MAPE (for wards with at least 1 birth per year occurring)
Ethnic group White
Ethnic group Indian
ethnic group: White
0
50
100
150
200
250
300
350 The 'combined best method' rates specific for each ethnic group (smoothed by Hadwiger function)
The 'grouping' rates specific for each ethnic group (smoothed by Hadwiger function)
Ward only rates (smoothed by Hadwiger function)
Ethnic group only rates (smoothed by Hadwiger function)
Bradford district rates
GAD England rates
ethnic group:Indian
0
50
100
150
The 'combined best method' rates specific for each ethnic group (smoothed by Hadwiger function)The 'grouping' rates specific for each ethnic group (smoothed by Hadwiger function)Ward only rates (smoothed by Hadwiger function)Ethnic group only rates (smoothed by Hadwiger function)Bradford district ratesGAD England rates
Comparing projected births to actual births for period 1991-1998 using the MAPE (averaged over the number
of wards, with at least 1 birth per year)MAPE (30 wards)
‘combined best method’
‘grouping’ rates
Ward only rates
Ethnic group only rates
Bradford district rates
GAD England rates
Mean 14.3 15.0 35.3 21.9 33.3 26.1 Median 10.0 9.5 14.3 13.4 24.3 17.1 Range 68.7 68.1 338.4 217.6 262.7 243.4 Minimum 3.6 4.3 4.5 5.2 3.8 4.7 Maximum 72.4 72.4 342.9 222.8 266.5 248.1
Ethnic group White
Ethnic group Black
Ethnic group Indian
MAPE (7 wards)
‘combined best method’
‘grouping’ rates
Ward only rates
Ethnic group only rates
Bradford district rates
GAD England rates
Mean 97.1 97.1 194.2 106.3 154.2 144.9 Median 54.6 54.6 185.6 64.0 110.7 107.3 Range 205.3 205.3 466.2 238.3 364.7 340.1 Minimum 35.0 35.0 33.4 36.5 33.2 33.6 Maximum 240.3 240.3 499.6 274.8 397.9 373.7
MAPE (14 wards)
‘combined best method’
‘grouping’ rates
Ward only rates
Ethnic group only rates
Bradford district rates
GAD England rates
Mean 43.2 43.3 62.5 42.8 51.6 45.0 Median 35.0 31.7 56.9 34.0 43.8 38.8 Range 95.9 95.9 112.1 84.5 102.4 94.7 Minimum 12.3 12.3 22.6 11.2 15.8 11.7 Maximum 108.2 108.2 134.7 95.7 118.2 106.3
Comparing projected births to actual births for period 1991-1998 using the MAPE (averaged over the number
of wards, with at least 1 birth per year)
Ethnic group Pakistani
Ethnic group Bangladeshi
Ethnic group Other
MAPE (19 wards)
‘combined best method’
‘grouping’ rates
Ward only rates
Ethnic group only rates
Bradford district rates
GAD England rates
Mean 26.0 27.1 48.4 25.1 52.2 56.6 Median 18.3 18.7 49.5 22.2 55.0 59.4 Range 75.4 71.2 37.6 47.6 30.3 30.7 Minimum 4.9 9.0 28.7 5.4 32.6 36.2 Maximum 80.2 80.2 66.3 53.0 62.9 67.0
MAPE (6 wards)
‘combined best method’
‘grouping’ rates
Ward only rates
Ethnic group only rates
Bradford district rates
GAD England rates
Mean 38.5 38.5 48.6 37.9 53.0 56.6 Median 36.8 36.8 47.3 40.4 55.5 59.5 Range 38.9 38.9 28.1 31.3 30.8 37.4 Minimum 22.2 22.2 35.7 19.0 33.0 31.7 Maximum 61.2 61.2 63.8 50.3 63.9 69.1
MAPE (22 wards)
‘combined best method’
‘grouping’ rates
Ward only rates
Ethnic group only rates
Bradford district rates
GAD England rates
Mean 56.9 56.9 64.0 59.8 57.9 58.0 Median 47.0 47.0 56.7 50.2 54.5 58.1 Range 94.7 94.7 155.1 141.5 97.5 77.9 Minimum 29.8 29.8 23.5 21.3 27.9 33.0 Maximum 124.5 124.5 178.6 162.8 125.4 111.0
Considering the results at the ward-level only (for period 1991-2000 from VS data)
No. of wards with less than 10% error
The ‘combined best method’
The ‘grouping’ rates
Ward only rates
Ethnic group only rates
Bradford
district rates
GAD England
ratesMAPE 22 21 24 19 11 12MPE 22 22 24 21 11 14
Conclusions for investigation
• The six different sets of rates were first considered by ethnic group at the ward-level. The results differed from what was expected; that is, for some ethnic groups, using the ethnic-specific fertility rates produced the smallest average MAPE.
• Second, when considering the errors at the ward-level, results were generally as expected, which is reassuring. Overall, using the ‘combined best method’, 22 of the wards were found to have both an MPE and MAPE of under 10%. It was also discovered that the method which produced the largest number of wards with both MPEs and MAPEs of under 10% (24 wards in total) was using ward-level ASFRs.
However, this finding is not the same as the one reported when the births were investigated by ethnic group at the ward level.
• In concluding on which method to recommend and when, it is very difficult to make an objective assessment of the findings considering all the different data limitations in both creating the ASFRs and assessing the projected births. For example, the revisions of the ONS population estimates from 1991 onwards, in the light of the 2001 Census results and problems in that the maternity records can be up to a few percent lower than the VS data for which they were compared to at the ward-level.
Conclusions for investigation
• In discussing more detailed subnational rates, with reference to the migration of the foreign born population in the US by regions, which was what the research paper was focused on, Rogers and Raymer comment:
“High levels of disaggregation permit a greater flexibility in the use of the projections by a wide variety of users; they also lead to a detection of greater consistency in patterns of behaviour among more homogeneous population subgroups. But greater disaggregation requires the estimation of even greater numbers of data points, both those describing population stocks and those defining the future rates of events and flows that are expected to occur. The practical difficulties of obtaining and interpreting such data soon outstrip the benefits of disaggregation”. Rogers and Raymer (1999)
