Nigeria - National Longitudinal Phone Survey 2021-2022, Phase 2

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.

ROUND 3: Interviewers attempted to contact and interview all 2,811 households that were successfully interviewed in the baseline (round 1) of the NLPS Phase 2, excluding 41 households that refused in Round 2. The third round of the NLPS Phase 2 also included individual-level data collection on employment and job history of household members. For the employment and job history modules, 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 4 household members were selected (in addition to the main respondent) to capture information on employment and job history. Therefore, for households with less than 4 adult members, all eligible members were included.

However, 90 percent of interviewed households had 4 or less adult members and only 10 percent had more than four. For the 10 percent with more than 4 adult members, 4 members were randomly selected from among the pool of eligible members. The selection was stratified by sex with an equal split of 2 male and 2 females was targeted, depending on the pool of eligible males and females. The selection of eligible household members in Round 3 was conditional to the selection conducted in Round 2 for the migration module. In that round, up to 6 household members were selected (15 years or older) to answer the migration module. However, the application of selection as relatively rare.

ROUND 4: Interviewers attempted to contact and interview all 2,852 households that were successfully interviewed in the baseline (round 1) of the NLPS Phase 2, excluding 70 households that refused in previous rounds of the survey.

ROUND 5: Interviewers attempted to contact and interview 2,824 households consisting of households that were successfully interviewed in the baseline (round 1) of the NLPS Phase 2 excluding 98 households that refused in previous rounds of the survey.


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).

ROUND 3: 2,694 households (93.5% of the 2,881 attempted) were contacted and 2,647 (91.9%) were successfully interviewed in the third round. Of those contacted, 21 households refused outright to be interviewed and 22 were partially interviewed. Of the 2,647 successfully interviewed households, 2,575 were households that have been successfully interviewed in all three rounds of the phase 2 survey so far. The third round of the NLPS Phase 2 also included individual-level data collection on employment and job history of household members. For the individual-level data collection, of 7,887 adult members of the household in round 3 (excluding the main respondent), information was collected on 5,942 or about 75.3 percent of eligible individuals. In addition, 2,665 main respondents were interviewed bringing the final sample of adult members with employment information to 8,597.

ROUND 4: 2,646 households (92.8% of the 2,852 attempted) were contacted and 2,605 (91.3%) were successfully interviewed in the fourth round. Of those contacted, 25 households refused outright to be interviewed and 8 were partially interviewed. Of the 2,605 successfully interviewed households, 2,431 were households that have been successfully interviewed in all four rounds of the phase 2 survey so far.

ROUND 5: 2,610 households (92.4% of the 2,824 attempted) were contacted and 2,574 (91.1%) were successfully interviewed in the fifth round. Of those contacted, 25 households refused outright to be interviewed and 5 were partially interviewed. Of the 2,574 successfully interviewed households, 2,319 were households that have been successfully interviewed in all five rounds of the phase 2 survey so far. These are the households that form a complete panel across the five rounds.


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:

w_ish=w_h x (n_hs/N_hs )^(-1)

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.

ROUND 3: In Round 3, several different weights are provided: two at the household-level and three at the individual-level. The two household weights provided are cross section and panel weights. The cross section weights are applicable to the entire round 3 sample while the panel weights are only applicable to round 3 sample households that have been successfully interviewed in all three rounds of phase 2 of the survey so far. For both of 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 section 2.2 (steps 4, 5 and 6). The round 3 household weights can be found in the household-level data file (p2r3_sect_a_2_5_6_6c_9a_12.dta) with the cross section weights in the variable named wt_p2round3 and the panel weights in the variable named wt_p2round3_panel.

Given the focus on individual employment and job history information in round 3 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 3 data. The individual weights for the employment and job history modules were calculated according to:

w_ish=w_h x (n_hs/N_hs )^(-1)

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_p2round3), 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_emp_p2r3 and can be found in the individual-level data file (p2r3_sect_2_6b_6c.dta). 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_emp_p2rr_pp_panel) contains the weight for individuals interviewed in the post-planting visit of the GHS-Panel wave 4 and the second (wt_emp_p2r3_ph_panel) contains the weight for individuals interviewed in the post-harvest visit of the GHS-Panel wave 4.

ROUND 4: In Round 4, two different household weights are provided: cross section and panel weights. The cross section weights are applicable to the entire round 4 sample while the panel weights are only applicable to round 4 sample households that have been successfully interviewed in all four rounds of phase 2 of the survey so far. For both of 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 section 2.2 (steps 4, 5 and 6). The round 4 household weights can be found in the household-level data file (p2r4_sect_a_2_5_5g_6_11a_11b_12.dta) with the cross section weights in the variable named wt_p2round4 and the panel weights in the variable named wt_p2round4_panel.

ROUND 5: In Round 5, two different household weights are provided: cross section and panel weights. The cross section weights are applicable to the entire round 5 sample while the panel weights are only applicable to round 5 sample households that have been successfully interviewed in all five rounds of phase 2 of the survey so far. For both of 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 section 2.2 (steps 4, 5 and 6). The round 5 household weights can be found in the household-level data file (p2r5_sect_a_2_5_6_9a_11b_13_12.dta) with the cross section weights in the variable named wt_p2round5 and the panel weights in the variable named wt_p2round5_panel.