# **Introduction**

This note serves as a guide to the calculation of weights for multiple waves of a household panel survey based on the experience of the Living Standards Measurement Study – Integrated Surveys on Agriculture (LSMS-ISA) project, located within the Development Research Group of the World Bank. The LSMSISA project is a multi-year project funded primarily by a grant from the Bill and Melinda Gates Foundation to collect panel data on households in Sub-Saharan Africa. The project is currently active in seven countries: Ethiopia, Malawi, Mali, Niger, Nigeria, Tanzania, and Uganda.

Although there is some variation between individual country projects, the main objective of the LSMSISA project is to provide public-use datasets to track income dynamics and changes in socioeconomic well-being with multiple rounds of data collection, while also providing as representative as possible a cross-section of households at each round. In the LSMS-ISA surveys, the initial rounds of all surveys were two-stage stratified cluster samples. Census enumeration areas (EAs) were selected in the first stage with probability proportional to size, then a household listing operation was conducted from which the second stage households were selected. As with all panel surveys, the LSMS-ISA surveys are only considered completely representative of the target population at the time of the selection of round 1. In subsequent rounds, the level of representativeness decays as the panel ages and there is sample attrition.<sup>2</sup> To minimize these effects, the LSMS-ISA surveys track both households that have changed location as well as a sub-sample of household members that have left households, surveying them in subsequent rounds in their new households. This methodology minimizes attrition by reducing the loss of shifted households, and allows new units to enter into the sample. With perfect implementation, the introduction of new households would match the natural changes in the overall cross-sectional population. In this case, the only loss of representativeness for the follow-up rounds would be from new households being formed by members not living in a particular country at the time of the round 1 survey.<sup>3</sup> This paper is primarily based on the experience of calculating the round 2 weights in the LSMS-ISA surveys from Tanzania and Uganda, and the round 3 weights in Uganda. The methodology described in this note builds upon published documentation from established panel surveys, such as the Panel Study of Income Dynamics [PSID], conducted since 1968 by the Institute for Social Research at the University of Michigan, and the British Household Panel Survey [BHPS], conducted since 1991 by the Institute for Social and Economic Research at the University of Essex ( <mark>Gouskova et al, 2008, Taylor, 2010)</mark> . Both the PSID and the BHPS are nationally-representative panel surveys in the USA and the UK respectively.

# **Calculating Weights for Round 2 of a Panel Survey**

The methodology for calculating weights for a panel survey is developed in the following eight steps:

- 1) Begin with the base weights (i.e. those calculated for round 1 of the survey) for round 2, or the ‘shadow weights’ for subsequent rounds (see section on the calculation of panel weights subsequent to round 2  for details on the calculation of shadow weights);

- 2) incorporate the probability of sub-selection of a round 1 unit into the round 2 sample; 3) incorporate the probability of sub-selection into tracking (if applicable); 4) derive fair-share factors for all household composition changes; 5) pool the weights in (1), (2) and (3) together;

> 2 It is also possible that the survey loses representativeness through changes in the population resulting from inmigration, although this is not believed to be a major threat in the LSMS-ISA survey context.

> 3 Exceptions to the general methodology described above include a rural-only sample design for the Ethiopia Rural Socio-Economic Survey, which also includes additional second-stage stratification, and not tracking split-off household members in the panel component of the Nigerian General Household Survey.

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