National Longitudinal Phone Survey 2021-2022, Phase 2
Other Household Survey [hh/oth]
The Nigeria National Longitudinal Phone Survey (NLPS) is a high-frequency phone survey of households, which was initially designed to follow the same households over time making it a powerful tool for studying and understanding the socio-economic impact of the COVID-19 pandemic in Nigeria.
From April 2020 to April 2021, the NBS has successfully implemented the Nigeria COVID-19 NLPS Phase 1 (https://microdata.worldbank.org/index.php/catalog/3712) with technical support from a World Bank team from the Development Data Group (DECDG) and the Poverty and Equity Global Practice. In Phase 1, the NBS conducted 12 rounds of monthly phone interviews with over 1,700 households that were selected from the General Household Survey-Panel (GHS-Panel) 2018/19. The extensive information collected in the GHS-Panel just over a year prior to the pandemic provided a rich set of background information on the Nigeria NLPS households which can be leveraged to assess the differential impacts of the health crisis in the country.
The objective of the Nigeria NLPS Phase 2 is to monitor in real-time how the Nigerian households are coping with national and global crises and their effects on the welfare and livelihoods of the households. The households in the Phase 2 are drawn from the sample of households interviewed in GHS-Panel 2018/19 including those interviewed during the Phase 1. This survey has become a flexible tool that contributes to filling critical gaps in information that could be used by the Nigerian government and stakeholders to help design policies to mitigate the negative impacts of the COVID-19 pandemic, the oil prices crises, inflation and global value chain crises, among others. The Nigeria NLPS Phase 2 is designed to accommodate the evolving nature of the crises, including revision of the questionnaire on a bi-monthly basis.
Kind of Data
Sample survey data [ssd]
Unit of Analysis
Version 01: Edited, anonymized dataset for public distribution
The Nigeria National Longitudinal Phone Survey 2021-2022 covered the following topics:
- Household Roster (Rounds 1, 2)
- Access to Health Services (Round 1)
- Employment (respondent) (Rounds 1, 2)
- Non-Farm Enterprise (Round 1)
- COVID Vaccination (Round 1)
- Migration (Round 2)
- Household Migrants Roster (Round 2)
The survey covered all de jure households excluding prisons, hospitals, military barracks, and school dormitories.
Producers and sponsors
National Bureau of Statistics (NBS)
Federal Government of Nigeria
The World Bank
Collaborated in the implementation of the survey
Bill and Melinda Gates Foundation
Funded the study
Federal Government of Nigeria
Funded the study
United States Agency for International Development
Funded the study
The World Bank
Funded the study
The Global Financing Facility for Women, Children and Adolescents
Funded the study
BASELINE (ROUND 1): Wave 4 of the GHS-Panel conducted in 2018/19 served as the frame for the Nigeria NLPS surveys. The GHS-Panel sample includes 4,976 households that were interviewed in the post-harvest visit of the fourth wave in January/February 2019. This sample of households is representative nationally as well as across the 6 geopolitical Zones that divide up the country. In every visit of the GHS-Panel, phone numbers are collected from interviewed households for up to 4 household members and 2 reference persons who are in close contact with the household in order to assist in locating and interviewing households who may have moved in subsequent waves of the survey. This comprehensive set of phone numbers as well as the already well-established relationship between NBS and the GHS-Panel households made this an ideal frame from which to conduct the NLPS in Nigeria.
Among the 4,976 households interviewed in the post-harvest visit of the GHS-Panel in 2019, 4,934 (99.2%) provided at least one phone number. Around 90 percent of these households (4,440) provided a phone number for at least one household member while the remaining 10 percent only provided a phone number for a reference person. For the second phase of the NLPS, all 4,440 GHS-Panel households with household member contact details were included in the sample to be contacted. This included the sample of households from the first phase of the NLPS who had household member contact details (2,701 of 3,000). Based on the response rate in the first phase of the NLPS of 65 percent, this was expected to yield an interviewed sample of nearly 2,900 households that is both nationally representative as well as representative of urban and rural areas of the country.
ROUND 2: Interviewers attempted to contact and interview all 2,922 households that were successfully interviewed in the baseline (round 1) of the NLPS Phase 2. The second round of the NLPS Phase 2 also included individual-level data collection on the migration history of household members. For the migration module, information on adult (15 years or older) members of the household was targeted, including respondents that fall into this age range. However, information was not captured for all adult members. In order to limit the burden for respondents and interviewers in cases where the number of adult members is large, a maximum of 6 household members were selected (in addition to the main respondent) to capture information on migration. Therefore, for households with less than 6 adult members, all eligible members were included. However, 93 percent of interviewed households had 6 or less adult members and only 7 percent had more than six. For the 7 percent with more than 6 adult members, 6 members were randomly selected from among the pool of eligible members. The selection was stratified by sex with an equal split of 3 male and 3 females was targeted, depending on the pool of eligible males and females. However, the application of selection as relatively rare.
BASELINE (ROUND 1): All 4,440 households were contacted in the first round of this second phase of the NLPS. 71 percent of sampled households were successfully contacted. Of those contacted, 93 percent or 2,922 households were fully interviewed. These 2,922 households constitute the final successful sample and will be contacted in subsequent rounds of the survey. Among those household that were contacted, only 121 refused to be interviewed (about 4% of successfully contacted households). Among the sample of households who were not successfully contacted, the predominant reason for noncontact was that the phone was switched off at every attempt (18% of all households). Wrong numbers and non-existent numbers were also present but less common affecting only 4.8 and 2.5 percent of sampled households while for 2.6 percent of households the phone was ringing (and thus an active line and phone) but no one was answering.
ROUND 2: 2,797 households (95.7% of the 2,922 attempted) were contacted and 2,750 (94.1%) were successfully interviewed in the second round. Of those contacted, 36 households refused outright to be interviewed and 10 were partially interviewed. For the individual-level data collection, of 7,653 adult members of the household in round 2 (excluding the main respondent), information was collected on 7,058 or about 92.2 percent of eligible individuals. In addition, 2,750 main respondents were interviewed bringing the final sample of adult members with migration information to 9,808. Although interviewers made attempts to collect migration information directly from each individual selected, it proved exceedingly difficult to do so. As a result, only 34.6% of individual responses were obtained from the actual individual in question with the remaining 65.4% being collected via proxy (typically provided by the main respondent).
BASELINE (ROUND 1): In order to produce national estimates from the successfully interviewed sample, weights must be applied to the information provided by sampled households. Weights for the GHS-Panel serve as the basis for the Nigeria NLPS surveys, but the weights must be adjusted to reflect the selection and interviewing process. The weights for the Nigeria NLPS were calculated in several stages.
1) Begin with the GHS-Panel full sample household weights.
2) Apply an adjustment factor for the selection into the frame (GHS-Panel households that have contact details for a household member). A ratio adjustment was applied at the Zone-level (the strata for the GHS-Panel) to preserve the sum of household weights within each Zone between the full GHS-Panel sample and the NLPS frame.
3) Apply an adjustment for selection into the NLPS sample. The adjustment is a simple expansion factor that is the inverse of the selection probability from the frame for each sampled unit.
4) Apply an adjustment factor for non-contact of sampled households. This was again performed with a ratio adjustment at the Zone-level.
5) Apply an adjustment factor for non-response of contacted households through a ratio adjustment at the Zone-level.
6) Calibrate the weights (following adjustments 2-5) according to the properties of the full weighted GHS-Panel sample. This calibration step adjusts the weights such that the estimates obtained from the final NLPS sample will match the weighted means of the full GHS-Panel sample for specified characteristics. The calibration was performed using only information obtained from the GHS-Panel interview and thus will only reflect changes in the sample composition and not changes over time. The calibration applied here aims to correct for selection bias that is introduced at any point between identification of the frame and the final successfully interviewed sample. Selection bias is of particular concern in phone surveys since some segment of the population does not have access to a phone and there are more difficult barriers to successfully reach and interview households over the phone. The calibration was applied using the ReGenesees package in R. The characteristics included in the calibration were numerous, reflecting different dimensions of household socioeconomic status that were correlated with nonresponse. Characteristics include consumption expenditure, household size sex of household head, marital status of the household head, age of the household head, education of the household head, working status of the household head, asset ownership, access to electricity, improved water source, improved sanitation facilities, access to financial services, land ownership, agricultural activities, as well as demographic breakdown according to sex and 8 age groups (0-6, 7-14, 15-24, 25-34, 35-44, 45-54, 55-64, and 65 years and older). The weights were also applied to the total number of households in the population given by the GHS-Panel weights.
7) Trim the weights. Outlier weights were trimmed at the 1st and 99th percentiles using the ReGenesees package in R which adjusts the weights to given bounds while minimizing the deviation from the estimates obtained from the calibration in step 6.
In subsequent rounds of the survey, steps 4, 5, and 6 will be applied to the final baseline weights.
The baseline (round 1) weights are located in the household-level data file (p2r1_sect_a_2_5_6_9a_12.dta) under the variable name wt_p2round1.
ROUND 2: In Round 2, several different weights are provided: one at the household-level and three at the individual-level. The household weights are the same as was provided in previous round. For the household weights, the baseline (round 1 of phase 2) weights were adjusted for noncontact and nonresponse as well as calibrated following the same procedures outlined in Round 1 (steps 4, 5 and 6). The round 2 household weights can be found in the household-level data file (p2r2_sect_a_2_2a_2b_6_12) in the variable named wt_p2round2.
Given the focus on individual migration information in round 2 and the selection steps outlined above for the sample of adult members, an additional three individual-level weights were calculated and provided in the round 2 data. The individual weights for the migration module were calculated according to:
Where w_ih is the sampling weight for individual i who is sex s (male or female) in household h, w_h is the final household level weight (i.e., wt_p2round2), N_hs is the total number of eligible adult household members (15 years or older) of sex s in household h and n_hs is the equivalent number of selected eligible individuals in the household. The individual weights were then calibrated to correspond to the sex and age distribution of the total adult population according to the post-harvest visit of the GHS-Panel. The age groups considered in the calibration were 15-24, 25-34, 35-44, 45-54, 55-64, and 65 years or older, all further disaggregated by sex (male/female).
The basic individual weight described above is the cross section individual weight that considers all individuals that migration information was collected on. This weight is called wt_migr_p2r2 and can be found in the individual-level data file (p2r2_sect_2_2a). However, an additional two weights are provided for the panel of individuals interviewed in the GHS-Panel wave 4 and round 2 of the NLPS Phase II (i.e., excluding individuals added in any round of the NLPS). The first weight (wt_migr_p2r2_pp_panel) contains the weight for individuals interviewed in the post-planting visit of the GHS-Panel wave 4 and the second (wt_migr_p2r2_ph_panel) contains the weight for individuals interviewed in the post-harvest visit of the GHS-Panel wave 4.
Dates of Data Collection
Baseline (Round 1)
Data Collection Mode
Computer Assisted Telephone Interview [cati]
Data Collection Notes
ORGANIZATION OF FIELDWORK: Data were collected by trained NBS interviewers who individually made phone calls either from a dedicated call center established in NBS headquarters or from their respective homes. Interviewers were allowed to make calls from home due to capacity constraints in the call center as well as social distancing measures undertaken in the office. While interviewers would occasionally meet in the office, most correspondence with the interviewers was made through WhatsApp, phone and emails.
GIFTS TO HOUSEHOLD: As a show of appreciation for the households’ participation, all households that gave consent to be interviewed, were transferred 1000 Naira credit to their phones (even if their interviews are only partially completed). The transfers are made to successfully interviewed households in every round. Since some of the sampled households do not have personal phone numbers, they were interviewed via a reference person’s phone. These reference persons were also credited 1000 Naira credit to their phones when the households are successfully interviewed via their phones.
PRE-LOADED INFORMATION: Basic information on every household was pre-loaded in the CATI assignments for each interviewer. The information was pre-loaded to (1) assist interviewers in calling and identifying the household and (2) ensure that each pre-loaded person is properly addressed and easily matched to the most recent interviews. Basic household information (location, household head name, phone number, etc.) was pre-loaded. The list of individuals from the previous interview and their basic characteristics were uploaded. This helped maintain the panel of individuals and ensured the status of each individual in the subsequent round of the survey.
RESPONDENTS: Each round of the Nigeria NLPS Phase 2 has ONE RESPONDENT per household. The respondent was the household head or a knowledgeable adult household member. The respondent must be a member of the household. Interviewers were instructed to make every effort to reach the same respondent in subsequent rounds of the survey, in order to maintain the consistency of the information collected. However, in cases where the previous respondent was not available, interviewers would identify another knowledgeable adult household member to interview.
DATA MONITORING AND EVALUATION: As an additional aid to ensuring good quality data, extensive monitoring was performed throughout the fieldwork for each round of the survey. Two monitoring exercises were implemented during data collection. First, Survey Solutions’ audio recording functionality was activated for 25 percent of the sample. These interview recordings were audited by 3 trained monitors, though not all recorded interviewers were able to be reviewed due to personnel constraints. On a daily basis, the monitors will listen to these recordings and fill in a structured questionnaire with their observations on interviewer performance. The feedback from these audio audits are then filtered to the respective interviewers.
The second quality check implemented were call backs to contacted households. The call backs were conducted by trained interviewers who are not part of the main data collection interviewers. Each day, up to 36 households that were contacted by the interviewing team are called by these call back interviewers. The call back interviewers conduct a short interview with the household to confirm that the interviewer did indeed conduct the interview, that certain key elements were clearly stated to the respondent, that the interviewer conducted themselves in a professional manner, and other details on the interview process. Further, the call back team asked several time-invariant questions of the respondent to further confirm the interview was fully conducted and the interviewer captured the information correctly. Feedback from call backs were routed to the respective interviewers to improve on identified areas. Further, the call back interviewers also called households that were not successfully contacted by the main interviewer. In some cases, the call back interviewer was able to reach the household. In such cases, the case was sent back to the interviewer to conduct the interview.
As a result of these quality checks, some of the interviewers were dropped from participating in the survey. There were also regular check-ins to address questions and issues the interviewers might have.
National Bureau of Statistics
Federal Government of Nigeria
BASELINE (ROUND 1): One questionnaire, the Household Questionnaire, was administered to all households in the sample. The Household Questionnaire provides information on demographics; access to health services; employment and non-farm enterprise; and COVID-19 vaccine.
ROUND 2: One questionnaire, the Household Questionnaire, was administered to all households in the sample. The Household Questionnaire provides information on demographics; migration; employment; and household migrants.
COMPUTER ASSISTED TELEPHONE INTERVIEW (CATI): The Nigeria NLPS exercise was conducted using Computer Assisted Telephone Interview (CATI) techniques. The household questionnaire was implemented using the CATI software, Survey Solutions. The Survey Solutions software was developed and maintained by the Data Analytics and Tools Unit within the Development Data Group (DECDG) at the World Bank. Each interviewer was given two tablets, which they used to conduct the interviews. Overall, implementation of survey using Survey Solutions CATI was highly successful, as it allowed for timely availability of the data from completed interviews.
DATA COMMUNICATION SYSTEM: The data communication system used in the Nigeria NLPS Phase 2 was highly automated. Each interviewer was given a mobile modem allowing for internet connectivity and daily synchronization of their tablet. This ensured that head office in Abuja has access to the data in real-time. Once the interview is completed and uploaded to the server, the data is first reviewed by the Supervisors, and then routed for call back or audio audit if selected. A feedback questionnaire was also designed in Survey Solutions where interviewers receive respective feedback on their tablet from the various monitoring stages. This activity is done on a daily basis throughout the duration of the data collection.
DATA CLEANING: The data cleaning process was done in three main stages. The first stage was to ensure proper quality control during the fieldwork. This was achieved in part by incorporating validation and consistency checks into the Survey Solutions application used for the data collection and designed to highlight many of the errors that occurred during the fieldwork.
The second stage cleaning involved the use of Supervisors in Survey Solutions. As indicated above, once the interview is completed and uploaded to the server, the Supervisors reviewed completed interviews for inconsistencies and extreme values. Depending on the outcome, they can either approve or reject the case. If rejected, the case goes back to the respective interviewer’s tablet upon synchronization. The supervisor will provide general and question-specific comments when rejecting a particular completed interview. These errors were then corrected based on another call to the household on the instruction of the supervisor. The data that had gone through this first stage of cleaning and has no issues is then approved by the Supervisor.
The third stage of cleaning involved a comprehensive review of the final raw data following the first and second stage cleaning. Every variable was examined individually for (1) consistency with other sections and variables, (2) out of range responses, and (3) formatting. Some minor errors remain in the data where the diagnosis and/or solution were unclear to the data cleaning team.
CONFIDENTIAL INFORMATION: For purposes of maintaining the confidentiality of the data, all names, phone numbers, and addresses have been removed from the datasets.
Biyi Fafunmi, Head of Department - ICT
National Bureau of Statistics
LSMS Data Manager
The World Bank
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National Bureau of Statistics. Nigeria National Longitudinal Phone Survey (NLPS) 2021-2022. Dataset downloaded from [source] on [date].