Nigeria - General Household Survey 2009, Fifth Round

The General Household Survey and the National Agricultural Sample Survey designs derived from NBS 2007/12 NISH sample design.
The 2007/12 NISH sample design is a 2-stage, replicated and rotated cluster sample design with Enumeration Areas (EAs) as first stage sampling units or Primary Sampling Units (PSUs) while Households constituted the second stage units (secondary sampling units). The households were the Ultimate Sampling Units for the multi-subject survey.

Generally, the NISH Master Sample in each State is made up of 200 EAs drawn in 20 replicates. A replicate consists of 10 EAs. Replicates 10-15, subsets of the Master Sample were studied for modules of the NISH.

The GHS was implemented as a NISH module. three replicates were studied per State including the FCT, Abuja. With a fixed-take of 15 HHs systematically selected per EA, 450 HHs thus were selected for interview per State including the FCT, Abuja. Hence, nationally, a total of 16,650 HHs were drawn from the 1,110 EAs selected for interview for the GHS. The selected EAs (and hence the HHs) cut across the rural and urban sectors.


Variance Estimate (Jackknife Method)
Estimating variances using the Jackknife method will require forming replicate from the full sample by randomly eliminating one sample cluster [Enumeration Area (EA) at a time from a state containing k EAs, k replicated estimates are formed by eliminating one of these, at a time, and increasing the weight of the remaining (k-1) EAs by a factor of k/(k-1). This process is repeated for each EA.

For a given state or reporting domain, the estimate of the variance of a rate, r, is given by
k
Var(r ) = (Se)2 = 1 S (ri - r)2
k(k-1) i=1

where (Se) is the standard error,
k is the number of EAs in the state or reporting domain.

r is the weighted estimate calculated from the entire sample of EAs in the state or reporting domain.
ri = kr - (k - 1)r(i), where

r(i) is the re-weighted estimate calculated from the reduced sample of k-1 EAs.

To obtain an estimate of the variance at a higher level, say, at the national level, the process is repeated over all states, with k redefined to refer to the total number of EAs (as opposed to the number in the states).


At National level ,out of the expected 1,110 EAs, all were covered which showed 100% retrieval rate. (by the table 1.12 on page 196 of the report)

At household level, out of the 16,650 expected to be covered, 16,355 were canvassed which showed 98% retrieval.

At sector level (Urban/Rural), 28.4% were recorded for Urban while Rural recorded 71.6%.


The variable (Hweight ) Household weight is computed and attached to the data file.

The formula adopted in calculating the design weights for the survey data (sample results) were as follows:
(i) The probability of selecting an EA within a state was obtained by dividing the total number of EAs sampled in a state by total number of EAs in that particular state. Let this be represented by fj. That is,
fj = Total Number of EAs sampled in a state
Total Number of EAs in that particular State

(ii) Likewise, the probability of selecting an housing unit (HU) within an EA was obtained by dividing the total number of housing units selected in an EA by the total number of housing units (HUs) listed in that particular EA. Let this be represented by fk. That is,
fk = Total Number of HUs selected in an EA
Total Number of HUs listed in that particular EA
Mathematically,
Design weight = Total number of EAs in a state
Total number of EAs sampled in that particular state
X
Total Number of HUs listed in an EA
Total Number of HUs selected in that particular EA

Estimation Procedures:
Let the probability of selecting the EA be fj and the probability of selecting the housing unit be fk. Then the product f = fjfk = 1 where fj = n and fk = h
Wj k N H.

Household Weight (HHWeight )


n h
Ys = N ? H ? X sj k
n j=1 h k=1

n h
= N H ? ? X sj k
n h j=1 k=1

n h
= W s j k ? ? X sj k (Note: W s j k = N . H )
j = 1 k=1 n h
Where:
ˆ
Ys = Estimate for states
N = Total Number of EAs in states
n = Selected number of EAs in states
H = Total number of Housing Units listed in the jth EA
h = Selected number of Housing Units in the jth EA.
Xsj k = Value of the element in the kth housing unit of jth EA in states.
Wsjk = Weight of the element in kth housing unit of the jth EA in states.