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Comparing network and association models in the analysis of historical patterns of occupational interactions and stratification Paul Lambert 1 , David Griffiths 1 , Richard Zijdeman 2 , Ineke Maas 2 , Marco van Leeuwen 2 Paper presented to the European Social Science History Conference, 11-14 April 2012, University of Glasgow, UK 1) University of Stirling, UK, contact email: [email protected] 2) University of Utrecht, Netherlands 1

Comparing network and association models in the analysis of historical patterns of occupational interactions and stratification Paul Lambert 1, David Griffiths

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Page 1: Comparing network and association models in the analysis of historical patterns of occupational interactions and stratification Paul Lambert 1, David Griffiths

Comparing network and association models in the analysis of historical

patterns of occupational interactions and stratification

Paul Lambert1, David Griffiths1, Richard Zijdeman2, Ineke Maas2, Marco van Leeuwen2

Paper presented to the European Social Science History Conference, 11-14 April 2012, University of Glasgow, UK

1) University of Stirling, UK, contact email: [email protected]) University of Utrecht, Netherlands

1

Page 2: Comparing network and association models in the analysis of historical patterns of occupational interactions and stratification Paul Lambert 1, David Griffiths

Motivation• Studying social interactions and social connections can

help us to understand social trends and transformations • Social mobility; homogamy; industrialisation; etc

• Taking full advantage of historical occupational codes, new data, and new analytical opportunities

• HISCO/NAPPHISCO/Microclass standardised codes…• …capture fine-grained details, but potentially aggregate

some occupations by sector rather than level

– GB 1831 census “..occupational returns as ‘crude, undigested, and essentially unscientific’, a document ‘whose insufficiency is a national disgrace to us, for there the trading and working classes are all jumbled together in the most perplexing confusion, and the occupations classified in a manner that would shame the merest tyro’” [Thompson 1963: 25, citing Mayhew 1862] 2

Page 3: Comparing network and association models in the analysis of historical patterns of occupational interactions and stratification Paul Lambert 1, David Griffiths

What’s new?1) Data resources• Census returns with household sharers’

occupations as proxy for social distance

0.1

.2.3

.4

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 20-30

USA, 1900, N=282158Number of jobs within household

% of cases % of jobs female

0.2

.4.6

1 2 3 4 5 6 7 8 9 10 12

Iceland, 1801, N=9071Number of jobs within household

% of cases % of jobs female

2) Occupational coding Originally in NAPP/PUMS codes (NAPPHISCO, or national unit)

(Approximate) recode into HISCO R Zijdeman; www.geode.stir.ac.uk(Approximate) recode into ‘Microclass’ D Griffiths; www.geode.stir.ac.uk

‘Microclass’ (Weeden and Grusky 2005; Jonsson et al. 2009) – socially defined fine-grained occupational clusters 3

Page 4: Comparing network and association models in the analysis of historical patterns of occupational interactions and stratification Paul Lambert 1, David Griffiths

Data sourcesCountry Years N cases (k) Source Occupations Derived Occs

Canada 1871, 1881, 1891, 1901

8; 1276; 156; 92

NAPP NAPPHISCO HISCO; Microclass

Iceland 1801, 1901 9; 34 NAPP NAPPHISCO HISCO; Microclass

Sweden 1900 1573 NAPP NAPPHISCO HISCO; Microclass

Britain 1851; 1881s; 1881ew

214; 2096; 13500;

NAPP OCCGB Microclass

USA 1850, 1860, 1870, 1880, 1900

53; 83; 121; 170, 282

PUMS US1880 HISCO; Microclass

Norway 1801, 1865, 1875,1900

228; 633; 286; 1037

NAPP NAPPHISCO HISCO; Microclass

N refers to number of adults in dataset with valid occupational records. The number of unique within household connections between these adults is usually between 1 and 2 times the number of adults.

4

Page 5: Comparing network and association models in the analysis of historical patterns of occupational interactions and stratification Paul Lambert 1, David Griffiths

1101. Jurists1102. Health professionals

1103. Professors and instructors1104. Natural scientists

1105. Statistical and social scientists1106. Architects

1107. Accountants1108. Journalists, authors, and related writers

1109. Engineers1201. Officials, government and non-profit organizations

1202. Managers1203. Commercial Managers

1204. Building managers and proprietors1304. Elementary and secondary school teachers

1305. Librarians1306. Creative artists

1307. Ship officers1308. Professional, technical, and related workers, n.e.c.

1309. Social and welfare workers1310. Workers in religion

1311. Nonmedical technicians1312. Health semiprofessionals

2001. Proprietors3101. Real estate agents

3102. Other agents3105. Sales workers and shop assistants

3201. Telephone operators3202. Bookkeepers and related workers

3203. Office and clerical workers3204. Postal and mail distribution clerks

4101. Craftsmen and kindred workers, n.e.c.4102. Foremen

4104. Printers and related workers4105. Locomotive operators

4106. Electricians4107. Tailors and related workers

4109. Blacksmiths and machinists4110. Jewelers, opticians, and precious metal workers

4111. Other mechanics4112. Plumbers and pipe-fitters

4113. Cabinetmakers4114. Bakers

4115. Welders and related metal workers4116. Painters

4117. Butchers4118. Stationary engine operators

Bricklayers, carpenters & related4120. Heavy machine operators

4202. Chemical processors4203. Miners and related workers

4204. Longshoremen and freight handlers4205. Food processors

4206. Textile workers4207. Sawyers and lumber inspectors

4208. Metal processors4209. Operatives and kindred workers, n.e.c.

4210. Forestry workers4301. Protective service workers

4302. Transport conductors4304. Food service workers

4305. Mass transportation operators4306. Service workers, n.e.c.

4307. Hairdressers4309. Launderers and dry-cleaners

4310. Housekeeping workers4311. Janitors and cleaners

4312. Gardeners5101. Fishermen

5201. Farmers and farm managers5202. Farm laborers

9990. Members of armed forces

0 2000 4000 6000 8000

Men

Women

Canada 1891, Males and females by microclass units. (5201/5201 downweighted by factor of 5).

Preliminary versions – contemporary microclasses a convenient way to measure and analysis fine-grained historical detail?

5

Page 6: Comparing network and association models in the analysis of historical patterns of occupational interactions and stratification Paul Lambert 1, David Griffiths

Sample Model CAM/USC Microclass HISCO NAPPHISCO (OCCGB)

CA 1871 R2 in predicting 0.155 0.247 0.270 0.303

CA 1881 alter’s HISCAM 0.194 0.279 0.309 0.310

CA 1891 0.299 0.404 0.433 0.437

CA 1901 0.143 0.252 0.280 0.283

IC 1801 R2 in predicting 0.060 0.137 0.166 0.167

IC 1901 alter’s HISCAM 0.009 0.032 0.043 0.043

SE 1900 ` ` 0.000 0.167 0.192 0.192

GB 1851 R2 in predicting 0.300 0.319 n/a 0.344

GB 1881 (EW) alter’s CAMSIS 0.236 0.258 n/a 0.282

GB 1881 (S) 0.189 0.228 n/a 0.245

US 1850 R2 in predicting 0.027 0.053 0.057 0.058

US 1860 alter’s literacy 0.026 0.059 0.065 0.066

US 1870 (plus father’s hiscam 0.067 0.145 0.151 0.151

US 1880 If literacy missing) 0.040 0.099 0.103 0.104

US 1900 0.032 0.069 0.075 0.076

NO 1801 R2 in predicting 0.067 0.115 0.156 0.157

NO 1865 alter’s HISCAM 0.028 0.064 0.081 0.081

NO 1875 0.057 0.099 0.116 0.117

NO 1900 0.084 0.162 0.180 0.1816

Page 7: Comparing network and association models in the analysis of historical patterns of occupational interactions and stratification Paul Lambert 1, David Griffiths

What’s new?3) Methods for analysing {within-household} social

connections on large-scale and fine-grained data

…Focus on the individual outcome.. Model with occupation-based indicators

(plus random or fixed effects)

…Focus on the social connection.. Association models

• HISCAM (Lambert et al. 2012)• Chan (2010) on ‘status’ scales

Network analysis• ‘SONOCS’ (Griffiths & Lambert 2011) • Cf. Wellman & Berkowitz (1988)

Characterise dimensions to the occupational

interaction structure

Identify particular ‘routes’ of

occupational connects 7

Page 8: Comparing network and association models in the analysis of historical patterns of occupational interactions and stratification Paul Lambert 1, David Griffiths

9999 5202 5201 5101 4312 4310 4304 4301 4209 4206 4205 4120 4119 4116 4115 4114 4113 4110 4109 4107 4104 4102 3204 3203 3202 3102 2001 1312 1310 1308 1307 1306 1304 1202 1201 1108 1105 1104 1103 1102 1101

1101

1102

1103

1104

1105

1108

1109

1201

1202

1304

1305

1306

1307

1308

1310

1312

2001

3102

3202

3203

3204

4102

4104

4107

4109

4110

4113

4114

4115

4116

4119

4120

4205

4206

4208

4209

4301

4304

4306

4310

4311

5101

5201

5202

9999

(Excluding diagonals)

Iceland, 1901Microclasses

8

Page 9: Comparing network and association models in the analysis of historical patterns of occupational interactions and stratification Paul Lambert 1, David Griffiths

9999 5202 5201 5101 4312 4310 4304 4301 4209 4206 4205 4120 4119 4116 4115 4114 4113 4110 4109 4107 4104 4102 3204 3203 3202 3102 2001 1312 1310 1308 1307 1306 1304 1202 1201 1108 1105 1104 1103 1102 1101

1101

1102

1103

1104

1105

1108

1109

1201

1202

1304

1305

1306

1307

1308

1310

1312

2001

3102

3202

3203

3204

4102

4104

4107

4109

4110

4113

4114

4115

4116

4119

4120

4205

4206

4208

4209

4301

4304

4306

4310

4311

5101

5201

5202

9999

(Excluding diagonals and pseudo-diagonals)

Iceland, 1901Microclasses

9

Page 10: Comparing network and association models in the analysis of historical patterns of occupational interactions and stratification Paul Lambert 1, David Griffiths

9999 9990 5202 5201 5101 4312 4311 4310 4309 4307 4306 4305 4304 4302 4301 4210 4209 4208 4207 4206 4205 4204 4203 4202 4120 4119 4118 4117 4116 4115 4114 4113 4112 4111 4110 4109 4107 4106 4105 4104 4102 4101 3204 3203 3202 3201 3105 3102 3101 2001 1312 1311 1310 1308 1307 1306 1305 1304 1204 1203 1202 1201 1109 1108 1107 1106 1105 1104 1103 1102 1101

1101

1102

1103

1104

1105

1106

1107

1108

1109

1201

1202

1203

1204

1304

1305

1306

1307

1308

1310

1311

1312

2001

3101

3102

3105

3201

3202

3203

3204

4101

4102

4104

4105

4106

4107

4109

4110

4111

4112

4113

4114

4115

4116

4117

4118

4119

4120

4202

4203

4204

4205

4206

4207

4208

4209

4210

4301

4302

4304

4305

4306

4307

4309

4310

4311

4312

5101

5201

5202

9990

9999

(Excluding diagonals, n*20)

Norway, 1865Microclasses

10

Page 11: Comparing network and association models in the analysis of historical patterns of occupational interactions and stratification Paul Lambert 1, David Griffiths

9999 9990 5202 5201 5101 4312 4311 4310 4309 4307 4306 4305 4304 4302 4301 4210 4209 4208 4207 4206 4205 4204 4203 4202 4120 4119 4118 4117 4116 4115 4114 4113 4112 4111 4110 4109 4107 4106 4105 4104 4102 4101 3204 3203 3202 3201 3105 3102 3101 2001 1312 1311 1310 1308 1307 1306 1305 1304 1204 1202 1201 1109 1108 1107 1106 1105 1104 1103 1102 1101

110

1 1

102 1

105 1

108 1

201 1

202 1

203 1

204 1

304 1

305 1

306 1

307 1

308 1

312 2

001 3

102 3

105 3

201 3

203 3

204 4

102 4

104 4

107 4

109 4

110 4

111 4

113 4

114 4

115 4

116 4

117 4

119 4

120 4

202 4

203 4

204 4

205 4

206 4

207 4

208 4

209 4

210 4

301 4

304 4

305 4

306 4

307 4

309 4

310 4

311 4

312 5

101 5

201 5

202 9

990 9

999

(Excluding diagonals, n*20)

Norway, 1865, m-fMicroclasses

11

Page 12: Comparing network and association models in the analysis of historical patterns of occupational interactions and stratification Paul Lambert 1, David Griffiths

99900 98990 98620 98190 98130 98100 97145 97125 96100 95510 95490 95455 95420 95300 95140 95130 95000 94990 94960 94290 94220 94170 94160 93120 92690 92625 92110 91030 90100 89242 89210 89190 89160 88090 87370 87350 87340 87330 87105 84222 84190 84100 83990 83920 83220 83210 83110 82020 81930 81925 81230 81120 80360 80320 80310 80190 79920 79600 79430 79400 79200 79190 79100 78200 78100 77810 77660 77610 77510 77310 77120 76150 75710 75600 75500 75457 75100 75000 74990 74930 74400 73400 73210 73200 72725 72000 71300 71110 71105 64960 64100 63190 63110 62740 62510 62490 62460 62210 61110 59220 58940 58320 58300 57030 56010 55140 54060 54010 53210 51050 51040 51030 51020 45290 45220 45125 43200 41040 41025 39350 39110 37040 33990 33140 33120 33110 32110 31000 22520 22290 21970 21960 21950 21240 21220 21200 21100 20210 17320 17140 17000 16310 16150 15915 15900 14140 13000 12110 6710 6310 6100 4200 2210 2120 2000

111

0 2

000

212

0 2

210

420

0 6

100

631

0 6

710

121

10

130

00

141

40

159

15

161

50

163

10

170

00

171

40

173

20

202

10

211

00

212

00

212

20

212

40

219

50

219

60

219

70

222

90

225

20

310

00

321

10

331

10

331

20

331

40

339

90

370

40

391

10

391

50

393

50

410

25

410

40

432

00

432

20

443

20

451

25

452

20

452

90

510

20

510

30

510

40

510

50

532

10

540

10

540

60

551

40

560

10

570

30

583

00

583

20

589

40

592

20

611

10

622

10

624

60

624

90

625

10

627

40

631

10

631

90

641

00

649

60

711

05

711

10

720

00

727

25

732

00

732

10

734

00

744

00

749

30

749

90

750

00

751

00

754

57

755

00

756

00

756

35

757

10

761

50

771

20

773

10

774

10

774

90

775

10

776

10

776

60

778

10

781

00

782

00

791

00

791

90

794

00

794

30

796

00

799

20

799

30

801

90

803

20

811

20

812

08

812

30

819

25

819

30

820

20

831

10

832

10

832

20

839

20

839

90

841

00

841

90

842

22

871

05

873

30

873

40

873

50

873

70

874

50

880

90

891

90

892

10

892

42

901

00

910

30

921

10

926

25

931

20

941

60

941

70

942

20

942

90

949

60

949

90

950

00

951

40

953

00

954

20

954

55

954

90

955

10

959

25

961

00

971

25

971

52

981

00

981

30

981

90

986

20

989

90

999

00

(n*4)

USA, 1850, m-m HISCO units

12

Page 13: Comparing network and association models in the analysis of historical patterns of occupational interactions and stratification Paul Lambert 1, David Griffiths

What can we do with such data?a) Statistical models of occupation-based outcomes b) Statistical models of the association processc) Network depictions of prevalence of connections

Intergenerational HISCAM (all m-m) R

Canada 1871=0.57; 1881=0.47; 1891=0.46; 1901=0.43

Iceland 1801=0.41, 1901=0.07

Sweden 1900=0.37

Britain 1851=0.21; 1881ew=0.36; 1881s=0.30

USA 1850=0.30; 1860=0.33; 1870=0.33; 1880=0.31; 1900=0.33

Norway 1801=0.23; 1865=0.23; 1875=0.29; 1900=0.27

4060

8010

0S

on

40 60 80 100Father

Values Perfect fit Regression fit

Father-Son Social Mobility [HISCAM, microclass, N=45k]Canada 1891

13

Page 14: Comparing network and association models in the analysis of historical patterns of occupational interactions and stratification Paul Lambert 1, David Griffiths

(a) Model individual outcomes: Linear/random/fixed effects

(1) (2) (3) (4) (5) (6)

OLS (1)+fath HISCAM

(2) + f.e. HISCO

(2) + f.e. microclass

(2) + r.e. HISCO

(2) + r.e. microclass

Age (linear) 29.5 32.1 35.7 34.6 35.7 34.5

Female -120.9 -127.2 -128.6 -130.1 -128.8 -130.1

Jewish 7.9 7.5 7.1 7.0 7.1 7.0

Sami 1.6 1.8 2.2 2.1 2.2 2.1

Finnish -2.0 -1.7 -1.7 -1.9 -1.7 -1.9

Urban 36.6 32.3 18.7 19.6 19.0 19.8

Cohabits -19.6 -18.5 -16.5 -17.0 -16.5 -17.0

Father’s HISCAM

37.5 5.4 3.6 6.5

Rho 0.197 0.038 0.086 0.026

r2 0.109 0.119

Data: Sweden 1900, N=124238, Child HISCAM predicted by father’s HISCAM. T-statistics.

14

Page 15: Comparing network and association models in the analysis of historical patterns of occupational interactions and stratification Paul Lambert 1, David Griffiths

(b) Association models‘Cambridge Social Interaction and Stratification Scales’ See www.camsis.stir.ac.uk/hiscam & Lambert et al. (2012) for historical data e.g.s

• Social Interaction Distance (‘SID’) analysis

• RC(II) model / Correspondence analysis

• First dimension of association can usually be labelled as ‘stratification’

15

Page 16: Comparing network and association models in the analysis of historical patterns of occupational interactions and stratification Paul Lambert 1, David Griffiths

How to use SID analysis effectively..?• Carefully prepared

specific analysis… • ..or semi-automated

comparisons?

• Fine- v’s coarse-grained analysis?

Scales scores can indicate change in occupations through context

Model fit statistics allow study of trends/structures

Fully automated , m-f homogamy, %inertia in dims 1+2

Fully automated , father-son, correlation to contemporary CAMSIS

Canada 1871=0.90; 1881=0.63; 1891=0.51; 1901=0.47

1871=0.38; 1881=0.44; 1891=0.56; 1901=0.64

Iceland 1801=0.94, 1901=0.73 1801=0.76, 1901=0.22

Sweden 1900=0.56 1900=0.11

Britain 1851=0.48; 1881ew=0.56; 1881s=0.53

1851=0.66; 1881ew=0.66; 1881s=0.10

USA 1850=. ; 1860=0.55; 1870=0.67; 1880=0.53; 1900=0.50

1850=0.01; 1860=0.16; 1870=0.03; 1880=0.12; 1900=0.62

Norway 1801=0.87; 1865=0.78; 1875=0.58; 1900=0.64

1801=0.68; 1865=0.49; 1875=0.65; 1900=0.20

16

Page 17: Comparing network and association models in the analysis of historical patterns of occupational interactions and stratification Paul Lambert 1, David Griffiths

0 20 40 60 80 100

52015101421042034113410199994107410943094204411543054205411041021301130913081307130511091107

Automated scales, selected scores onlyNorway, 1900 and 1801

Male score, 1900 Male score, 1801

• Main contribution of association models are to tell us about average social positions of the incumbents of occupations (and change over societies) 17

Page 18: Comparing network and association models in the analysis of historical patterns of occupational interactions and stratification Paul Lambert 1, David Griffiths

Canada Norway Scotland USA

Cases 123,749 54,067 261,187 22,349

Links 101 136 111 208

Microclasses (older cohort) 45 50 41 45

Microclasses (younger cohort) 35 38 39 41

Strongest bond (* times expectation) 239 146 19 55

Network: Degree centrality .10 .14 .12 .18

Network: Closeness centrality .23 .23 .27 .26

Network: Components 2 1 2 1

Network: Distance 10 12 7 5

Network: average distance 3.8 3.7 3.2 2.6

Note, for Canada and Scotland closeness centrality refers to largest component only.

c) Network analysis

Still looking at number of connections {within household} but change in emphasis on features of connections

Page 19: Comparing network and association models in the analysis of historical patterns of occupational interactions and stratification Paul Lambert 1, David Griffiths

Canada 1881

USA 1880

Scotland 1881

Norway 1876

Microclasses with ties *2 expected + non-sparse; male-male links if >16yrs age gap

Page 20: Comparing network and association models in the analysis of historical patterns of occupational interactions and stratification Paul Lambert 1, David Griffiths

Scotland 1881

Librarians (1305) and creative artists (1306) with links to printers (4104) and craftsmen

Housekeepers (4310)

Farming community (5201, 5202), forestry workers (4210) and gardeners (4312)

Managers (1202) and ships’ officers (1307) link to their subordinates (4306)

Clerks (3203) and agents (3102) interact with various professionals

Lawyers (1101), medics (1102), teachers (1304) and the clergy (1310) form a clique at centre of the network

Page 21: Comparing network and association models in the analysis of historical patterns of occupational interactions and stratification Paul Lambert 1, David Griffiths

Canada 1881

Ties not as obvious; sparse connections within mesoclasses, but stratification effects most observable

Farmers (5201) and farm labourers (5202) do not have mutual ties to forestry workers

Teachers (1304), clergy (1310), lawyers (1101) and medics (1102) have sparse ties

Clerical and sales workers (3***) strongly interact, but few ties to professionals (1***)

Librarians (1305) and creative artists (1306) don’t form any strong ties and aren’t represented

Food service workers (4304) are the ‘sons’ of many other routine workers

Housekeepers (4310)

Page 22: Comparing network and association models in the analysis of historical patterns of occupational interactions and stratification Paul Lambert 1, David Griffiths

Canada 1881 (left) with microclasses split by religion (red=catholic; white=non-catholic).

Clear division on religious grounds in 1881.

Canada 1891 (right) with microclasses split by religion (red=catholic; white=non-catholic).

Religious divide continues, but much more common for cross-religion and microclass households.

Page 23: Comparing network and association models in the analysis of historical patterns of occupational interactions and stratification Paul Lambert 1, David Griffiths

Canada (by religion) 1881 1891

Cases 92,048 22,084

% Roman Catholic 33.1% 28.6%

% Catholics with Catholic alter 84.1% 60.6%

% non-Catholics with Catholic alter 8.2% 17.4%

Mean HISCAM (All cases)(Standard deviation)

58.0 (10.9)

57.7 (11.4)

Mean difference in HISCAM (all cases)(Standard deviation)

9.2 (11.5)

10.1 (11.6)

% HISCAM difference<1/2 s.d.

…. (Catholic – Catholic) 52.0% 51.7%

… (non-Catholic to non-Catholic) 51.5% 49.3%

… (Catholic to non-Catholic) 45.5% 44.4%

% HISCAM difference>2 s.d.

… (Catholic to Catholic) 11.4% 16.6%

… (non-Catholic to non-Catholic) 12.8% 11.9%

… (Catholic to non-Catholic) 12.4% 11.8%

Page 24: Comparing network and association models in the analysis of historical patterns of occupational interactions and stratification Paul Lambert 1, David Griffiths

Summary: Social connections between occupations

• Connections are central to social organisation of the stratification system [e.g. Bottero 2005]

• Problems of data preparation and scale• Occupational coding – NAPP; HISCO; Microclass • Identify social connections (within hhld NAPP)• Select/discard some types of connections (e.g. farming)

• Analytical approachesModel with proxy indicators, random or fixed effects…Focus on the social connection..Association models Network analysis

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Page 25: Comparing network and association models in the analysis of historical patterns of occupational interactions and stratification Paul Lambert 1, David Griffiths

References cited• Bottero, W. (2005). Stratification: Social Division and Inequality. London: Routledge.• Griffiths, D., & Lambert, P. S. (2011). Dimensions and Boundaries: Comparative analysis of occupational

structures using social network and social interaction distance analysis Paper presented at the ISA RC28 Spring meeting, University of Essex, 13-16 April 2011.

• Jonsson, J. O., Grusky, D. B., Di Carlo, M., Pollak, R., & Brinton, M. C. (2009). Microclass Mobility: Social Reproduction in Four Countries. American Journal of Sociology, 114(4), 977-1036.

• Lambert, P. S., Zijdeman, R. L., Maas, I., van Leeuwen, M. H. D., & Prandy, K. (2012). The construction of HISCAM: A stratification scale based on social interactions for historical research. Historical Methods, forthcoming.

• Mayhew, H. (1862) London Labour and the London Poor. • Thompson, E. P. (1980[1963]). The Making of25 the English Working Class. London: Penguin.• Weeden, K. A., & Grusky, D. B. (2005). The Case for a New Class Map. American Journal of Sociology,

111(1), 141-212.Data from: • Minnesota Population Center. (2011). Integrated Public Use Microdata Series, International: Version 6.1

[Machine readable database]. Minneapolis: University of Minnesota, and https://international.ipums.org/ (accessed 1 July 2011).

• North Atlantic Population Project and Minnesota Population Center. (2008). NAPP: Complete Count Microdata. NAPP Version 2.0 [computer files]. Minneapolis, MN: Minnesota Population Center [distributor] [http://www.nappdata.org]

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