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Food and You 2: Wave 6 Technical report

FY2 Wave 6 Technical report: Weighting

The same weighting approach was taken in Wave 6 as in previous waves.

Diweddarwyd ddiwethaf: 26 July 2023
Diweddarwyd ddiwethaf: 26 July 2023

Overview of weighting

The same weighting approach was taken in Wave 6 as in previous waves. Weights were initially calculated separately for each country in two stages:

1.    Calculation of selection weights (described in the section on selection weights)
2.    Calibration of selection weights to country population totals (described in section on population weights)

Next, weights were created for use in analyses of combined-country data by scaling the weighted country sample sizes to be proportional to the corresponding country population values (for adults aged 16 and over). 

Because it was not possible to include all questions in the postal questionnaires (see the section called ‘Questionnaire development and cognitive testing’), four separate question-type weights were calculated in each country, and in the combined all-country sample. These four question-type weights were designed to be used as follows:

  1. All-questionnaire weights to be used for questions asked of all sample members in all online and postal questionnaires
  2. Online questionnaire weights to be used for questions asked only of online participants (for example, questions not asked in the postal questionnaires)
  3. Online questionnaire plus Eating at Home (EH) postal questionnaire weights to be used for questions asked only of online participants and postal questionnaire respondents receiving only the EH version (for example, questions not asked in the version EO postal questionnaires)
  4. Online questionnaire plus Eating Out (EO) postal questionnaire weights to be used for questions asked only of online participants and postal questionnaire respondents receiving only the EO version (for example, questions not asked in the version EH postal questionnaires)

Four additional weights (one for each of these question types) were calculated for the combined all-country sample. Once the main weights were calculated as described above, supplementary ‘Wales & Welsh-England’ weights were calculated. These were designed to allow comparisons to be made between Wales and England (excluding London) after controlling for country profile differences in age within gender, ethnic group, number of adults per household, and urban-rural mix.

Calculation of selection weights

Selection weights were created to compensate for (i) variations in within-household individual selection probabilities and response propensities and (ii) the fact that, by design, some questions were not included in all questionnaires. As a maximum of two eligible adults were surveyed per household, adults in larger households are less likely to be included in the survey. So without this weight, individuals living in households in which some eligible adults were not interviewed would be underrepresented relative to individuals living in households in which all eligible adults were interviewed. They were calculated in the following stages: 

  1. The all-questionnaire selection weight was calculated as: (number of eligible people aged 16 years or over in the household)/(number of participants in the household). 
  2. The online questionnaire selection weight was calculated as: (number of eligible people aged 16 years or over in the household)/(number of online participants in the household). 
  3. Next the Online questionnaire plus version EH postal questionnaire weight and the online questionnaire plus version EO postal questionnaire weight were calculated by doubling the value of the all-questionnaire selection weight for postal respondents relative to the corresponding value for online respondents (because the relevant questions were only asked in half the postal questionnaires).    

Values were capped to the range 1-3 for the all-questionnaire and online selection weights, and to the range 1-6 for the online questionnaire plus version EH postal questionnaire and online questionnaire plus version EO postal questionnaire weights to restrict variance inflation.

Calibration to population values

Next, selection weights were applied to the three individual country samples and each was calibrated to the corresponding country population values for the number of adults aged 16 or over by: 

(i)    age band within gender 
(ii)    geographic area (defined separately for each country) 
(iii)    deprivation quintile (calculated using each country’s multiple deprivation index). 

These weighting variables are often used as standard in social surveys because they correlate reliably with both response propensity and a wide range of survey variables. We note that in some previous rounds of the face-to-face Food and You survey, working status was used as a weighting variable instead of deprivation quintile. In previous waves of Food and You 2 it was decided not to use this variable for weighting the sample because survey fieldwork took place during the Covid-19 pandemic, during which rates of employment were likely to be unstable.  Deprivation quintile was used as a substitute indicator of general economic prosperity. This approach was taken again for Wave 6, and is expected to continue for the immediate future for comparability.

Weighting targets are shown in the next section, taken from ONS Mid 2020 Population Estimates and NISRA Mid 2020 Population Estimates.

Table 9: Population totals for age within gender in England

Age band Males Females
16 to 24 3,066,029 2,884,608
25 to 29 1,924,416 1,847,077
30 to 34 1,916,412 1,908,240
35 to 39 1,852,969 1,885,240
40 to 44 1,730,268 1,746,035
45 to 49 1,803,208 1,835,431
50 to 54 1,911,318 1,964,033
55 to 59 1,852,593 1,909,189
60 to 64 1,568,489 1,628,324
65 to 69 1,347,714 1,436,586
70+ 2,278,001 2,546,119
All 1,1651,748 1,689,851

Table 10: Population totals for age within gender in Wales

Age band Males Females
16 to 24 180,657 164,947
25 to 29 106,877 101,237
30 to 34 98,579 98,093
35 to 39 91,729 94,144
40 to 44 85,290 87,640
45 to 49 93,969 98,496
50 to 54 105,438 111,532
55 to 59 108,081 114,141
60 to 64 95,915 101,501
65 to 69 87,314 92,641
70+ 149,473 163,436
All 71,981 103,745

Table 11: Population totals for age within gender in Northern Ireland

Age band Males Females
16 to 24 104,333 96,676
25 to 29 60,377 59,442
30 to 34 62,883 63,699
35 to 39 60,758 63,594
40 to 44 56,927 61,017
45 to 49 59,844 63,095
50 to 54 63,786 66,797
55 to 59 62,595 64,908
60 to 64 53,421 55,599
65 to 69 44,862 45,831
70+ 68,762 77,834
All 32,527 50,133

Table 12: Population totals for regions in England

England Region code England Region name Population total
E12000001 North East 2,203,353
E12000002 North West 5,957,266
E12000003 Yorkshire and The Humber 4,474,428
E12000004 East Midlands 3,963,265
E12000005 West Midlands 4,791,343
E12000006 East of England 5,051,203
E12000007 London 7,149,281
E12000008 South East 7,442,850
E12000009 South West 4,664,909
Total - 45,697,898

Table 13: Population totals for regions in Wales

Wales Region Population total
North 579,711
Mid 174,082
South West 586,562
South East 1,266,501
Total 2,606,856

Table 14: Population totals for regions in Northern Ireland

Northern Ireland (Local Government District code) Northern Ireland (Local Government District name) Population total
N09000001 Antrim and Newtonabbey 113,924
N090000011 Ards and North Down 132,057
N09000002 Armagh City, Banbridge and Craigavon 168,360
N09000003 Belfast 274,369
N09000004 Causeway Coast and Glens 116,337
N09000005 Derry City and Strabane 118,371
N09000006 Fermanagh and Omagh 91,929
N09000007 Lisburn and Castlereagh 116,887
N09000008 Mid and East Antrim 112,616
N09000009 Mid Ulster 114,153
N090000010 Newry, Mourne and Down 140,697
Total - 1,499,700

Table 15: Population totals for deprivation quintiles in England

England Quintiles 16+_Pop_20
1 9,138,329
2 9,140,152
3 9,139,700
4 9,139,337
5 9,140,380
Total

45,697,898

Table 16: Population totals for deprivation quintiles in Wales

Wales Quintiles 16+_Pop_20
1 521,330
2 520,803
3 521,290
4 522,008
5 521,425
Total

2,606,856

Table 17: Population totals for deprivation quintiles in Northern Ireland

NI Quintiles 16+_Pop_20
1 299,268
2 300,459
3 299,450
4 300,395
5 300,128
Total

1,499,700


Initial calibration was carried out separately in each country for each of the four questionnaire type weights described above. For each questionnaire type weight, calibration adjustment factors were calculated by dividing the individual country weights by the selection weights. These adjustment factors were then capped at the 99th percentile value to limit variance inflation and applied to selection weight to produce final individual country weights.  

After calibration and adjustment factor capping, the individual country level weights were scaled to equalise unweighted and weighted sample sizes in each country.

The aim of these within-country calibration procedures was to match the profile of the weighted sample to that of the population aged 16 or over on gender, age band, geographic region, and deprivation quintile.  In practice, there will be slight discrepancies between weighted sample totals and population figures as a result of the adjustment factor caps.

Creating of all country weight

An all-country version of each questionnaire type weight was then constructed by combining the individual country samples and rescaling final individual country weights so that weighted sample country proportions matched the respective country population (aged 16 years or over) proportions.

'Wales and Welsh England' standardisation weight

This weight was designed to calibrate English sample estimates to Welsh population characteristics for comparative purposes. It was calculated from the England sample as follows:

  1. London cases were dropped (London being in many ways unique in the UK)
  2. The non-London England sample proportions were calibrated to the weighted Wales sample proportions for four variables: number of adults in the household, ethnic group, urban-rural and age by gender. These four variables were selected when the ‘Wales & Welsh-England’ weights were first constructed in Wave 1. Weighted estimates for Wales and non-London England were compared across a range of candidate variables and statistically significant differences were found for urban-rural, ethnic group, household size and age within gender.

The final weighting variables were defined as follows:

Table 18: Age within gender (male and female)

Males Females
16 to 24 16 to 24
25 to 29 25 to 29
30 to 34 30 to 34
35 to 39 35 to 39
40 to 44 40 to 44
45 to 49 45 to 49
50 to 54 50 to 54
55 to 59 55 to 59
60 to 64 60 to 64
65 to 69 65 to 69
70+ 70+

Number of adults in household:

  • 1 adult
  • 2 adults
  • 3+ adults
  • Question not answered

Ethnic group:

  • White 
  • Asian
  • Black
  • Mixed
  • Other/not answered

Urban rural:

Urban: OA falls into a built up area with a population of 10,000 or more

Rural: All other OAs

‘Wales & Welsh-England’ weights were calculated only for respondents in England outside London and in Wales (where they were the same as the individual country weight for Wales). 

Table 18: Summary list of wights and when to use each one

Weight When to be used
wt1 Estimates for all-countries: questions asked of all sample members completing the online and postal questionnaires
wt2 Estimates for all-countries: questions asked only of online participants (not asked in postal questionnaire)
wt3 Estimates for all-countries: questions asked of all sample members completing the online questionnaires and those completing EH version of the postal questionnaire
wt4 Estimates for all-countries: questions asked of all sample members completing the online questionnaires and those completing EO version of the postal questionnaire
wt5 Individual country estimates for England, Wales and Northern Ireland: questions asked of all sample members completing the online and postal questionnaire
wt6 Individual country estimates for England, Wales and Northern Ireland: questions asked only of online participants (not asked in postal questionnaire)
wt7 Individual country estimates for England, Wales and Northern Ireland: questions asked of all sample members completing the online questionnaires and those completing EH version of the postal questionnaire
wt8 Individual country estimates for England, Wales and Northern Ireland: questions asked of all sample members completing the online questionnaires and those completing version EO of the postal questionnaire
wt9 ‘Wales and Welsh-England’ estimates: questions asked of all sample members in the online and postal questionnaire
wt10 ‘Wales and Welsh-England’ estimates: questions asked only of online participants (not asked in postal questionnaire)
wt11 ‘Wales and Welsh-England’ estimates: questions asked of all sample members completing the online questionnaires and those completing EH version of the postal questionnaire
wt12 ‘Wales and Welsh-England’ estimates: questions asked of all sample members completing the online questionnaires and those completing version EO of the postal questionnaire