9th Workshop on Labour Force Survey Methodology – Rome, 15-16 May 2014

The Italian LFS sampling design: recent and future developments

9th Workshop on Labour Force Survey Methodology

Rome, 15-16 May 2014

Loredana Di Consiglio Silvia LorigaAlessandro MartiniRita Ranaldi

9th Workshop on Labour Force Survey Methodology – Rome, 15-16 May 2014

Two-stages with

stratification of the PSUs (municipalities) in the first stage

rotation of the FSUs (households) in the last stage

IT-LFS sampling design /1

9th Workshop on Labour Force Survey Methodology – Rome, 15-16 May 2014

At the first stage …

Municipalities are stratified at NUTS III level according to resident population and they are divided into two groups:

SR (self-representative) municipalities:have larger demographic size (over a given threshold)each represents one stratum always selected in the sample

NSR (non self-representative) municipalities: have smaller demographic sizeare stratified in groups (strata) having almost the same total populationonly one municipality for each stratum is selected in the sample, with probabilities proportional to its demographic size

IT-LFS sampling design /2

9th Workshop on Labour Force Survey Methodology – Rome, 15-16 May 2014

At the second stage …

Households are randomly selected from the population registers of municipalities drawn at the first stage

FSUs are rotated according to a 2-(2)-2 rotation scheme. Households are interviewed during two consecutive quarters. After a two-quarters break, they are again interviewed twice in the corresponding two quarters of the following year

IT-LFS sampling design /3

9th Workshop on Labour Force Survey Methodology – Rome, 15-16 May 2014

Quarter 1 2003 A2 B1

Quarter 2 2003 B2 C1

Quarter 3 2003 C2 D1

Quarter 4 2003 A3 D2 E1

Quarter 1 2004 A4 B3 E2 F1

Quarter 2 2004 B4 C3 F2 G1

Quarter 3 2004 C4 D3 G2 H1

Quarter 4 2004 D4 E3 H2 I1

Quarter 1 2005 E4 F3 I2 L1

ROTATION GROUPREFERENCE

PERIOD

Sampling design: household rotation scheme

9th Workshop on Labour Force Survey Methodology – Rome, 15-16 May 2014

Leaving unchanged the general structure of the sample, in 2012 sampling design has been revised for the following main reasons:the previous sample was designed in 2001-2002, considering the target variables estimated at that time by the quarterly LFS and the frame information for stratification referred to 2002several changes occurred in the boundaries of the administrative units such as municipalities and provincesto further improve the monthly representativeness of the sample, considering the high relevance of monthly LFS estimatesbudget constraints forced to reduce the sample size

Sampling design revision /1

9th Workshop on Labour Force Survey Methodology – Rome, 15-16 May 2014

Comuni campione RCFL

Every quarterEvery quarter

71,536 71,536 theoretical theoretical sample sample households households (average (average sampling rate: sampling rate: 1/350)1/350)

Sample size after revision

allocatedallocatedin more than in more than 1,000 1,000 sample sample municipalities municipalities (about 1 out of 7)(about 1 out of 7)

9th Workshop on Labour Force Survey Methodology – Rome, 15-16 May 2014

The following methodological and operational constraints have been taken into account:Eurostat precision requirements (Reg.577/98), but also additional constraints for national purposes have been consideredthe unemployment figures considered as target variables for the evaluation of precision requirements are referred to the pre-crisis periodthe information on non responses has been considered when distributing the sample units among the territorial unitsthe monthly distribution of the sample guarantees that each month is representative of the whole national territorythe new selected PSUs have to overlap as more as possible with the previous PSUs in order to minimize the impact on the fieldwork (and on the final estimates)a random rotation of the PSUs has to be applied every year to maintain the sample updated over time

Sampling design revision /2

9th Workshop on Labour Force Survey Methodology – Rome, 15-16 May 2014

Distribution of the sample over spaceBecause of the national precision requirement about unemployment estimates in NUTS 3 domains, the distribution of the sample is not proportional to the demographic size of the domains

  NUTS 3 domainsITALY

MIN MEAN MAXResident households

(N)24,779 231,841 1,769,720 25,502,535

Unemployment rate % (2004-2007)

2.56 7.29 18.50 7.16

Sample size (n) 192 650 3,408 71,536

Inclusion probabilities (n/N%)

0.12 0.39 3.94 0.28

Minimum, mean value and maximum of resident population, unemployment rate, quarterly sample size and inclusion probabilities in NUTS 3 domains

The sample deviates from the optimal sample we should have obtained considering just Eurostat NUTS 2 constraints. In any case, Eurostat constraints are satisfied.

9th Workshop on Labour Force Survey Methodology – Rome, 15-16 May 2014

The quarterly sample size is uniformly distributed among the 13 weeks, each stratum is observed at least in 3 weeks per quarter and the monthly representativeness of the sample is guaranteed

The largest PSUs are in the sample all the 13 weeks of the quarter

Other PSUs (among them also some chief towns at NUTS3 level) are in the sample just 3 weeks per quarter, assigning them reference weeks that are triplet of weeks in which the distance between them is 4 weeks: 1-5-9 or 2-6-10 or 5-9-13 and so on

Distribution of the sample over time /1

9th Workshop on Labour Force Survey Methodology – Rome, 15-16 May 2014

The months are not fixed, but they are composed by a number of weeks that is variable (4 or 5) and depends on the number of Thursdays falling in each solar month

Distribution of the sample over time /2

Possible combinations

Weeks

1 2 3 4 5 6 7 8 9 10 11 12 134-4-5 4-5-4 5-4-4

Month 1Month 2Month 3

Week 5 may be included into months 1 or 2 and week 9 may be included into months 2 or 3

Some PSUs, to which the weeks 5 or 9 have been assigned, may fall into different months

In the sample revision we guaranteed that the chief towns in NUTS 3 domain, observed just 3 weeks per quarter, are not to be observed neither in week 5 neither in week 9

9th Workshop on Labour Force Survey Methodology – Rome, 15-16 May 2014

Aim: to minimize the impact on the fieldwork (changing all the municipalities, or the majority of them, would have meant to recruit and to train a lot of new interviewers, with evident effects on the fieldwork and risks on the quality of the final estimates)

Method: use of Permanent Random Numbers (PRN) applying the method suggested by Ernst (2004)

Results: 831 municipalities, about 75% of the PSUs selected according to the new design, overlapped with the previous PSUs

Maximum overlapping of old and new PSUs

9th Workshop on Labour Force Survey Methodology – Rome, 15-16 May 2014

Aim: to maintain the sample updated over time and to reduce the statistical burden, in particular for the households living in municipalities with a small number of residents

Method: Probabilistic rotation by applying permanent random numbers (PRN) and constant shift method (Brewer et al. 1972, Ohlsson 1995)

Results: in 2014, 143 municipalities have been rotated, about 13% of the PSUs that were sample in 2013, almost all the municipalities with less than 1,000 inhabitants and nearly three out of four municipalities of 1,001-2,000 inhabitants

Rotation of PSUs of small demographic size

9th Workshop on Labour Force Survey Methodology – Rome, 15-16 May 2014

The new sample has been gradually introduced starting from the first wave of 2012Q3. For 5 quarters, until 2013Q3, old and new sampling designs were overlapped

The estimation and analysis of variance procedures have been reviewed assuming that the two different sub-samples are independent

The comparability of the accuracy between the two designs is not simple for the wide variations in the estimates due to the current economic situation and to usual seasonal effects as well observed in this period of 15 months

The analysis was conducted using regression models that fit sampling errors, in order to obtain estimates of sampling errors independently by the observed phenomena, even with an approximate evaluations of the errors

Accuracy evaluation of the two sample designs /1

9th Workshop on Labour Force Survey Methodology – Rome, 15-16 May 2014

Accuracy evaluation of the two sample designs /2

Coefficient of Variation- IT-LFS Whole Country

0

5

10

15

20

25

30

35

40

5000 35000 65000 95000 150000 220000 340000 550000 850000 1300000 2000000

Old Sampling Design New Sampling Design

Graph 1 - Comparison of regression model of sampling errors for the new and old sample design of IT-LFS

9th Workshop on Labour Force Survey Methodology – Rome, 15-16 May 2014

Accuracy evaluation of the two sample designs /3Graph 2 - Difference of coefficient of variation between old and new IT-LFS sample design by Nuts II level

9th Workshop on Labour Force Survey Methodology – Rome, 15-16 May 2014

Istat has undertaken its process of renewal with the:transition to CAPI mode of several PAPI surveysintegration of the Trips and Holidays Survey as module into HBSintroduction of web in the surveys on PHD and on high school graduates

Carrying out several CAPI sample households surveys together with the new Population Rolling Census makes necessary to develop a coordinated approach to obtain harmonized sampling designs and to optimize the distribution of the sample over space and time

Future perspectives

9th Workshop on Labour Force Survey Methodology – Rome, 15-16 May 2014

Thank you for your attention