NDVI (Normalized Difference Vegetation Index) consists of a set of 72 variables. These variables report the maximum NDVI value within a 10-kilometer circular buffer around each DHS cluster for the 60 individual months prior to the survey start date, the month of the survey start date, and the 11 individual months following the survey start date.
NDVI is a numeric, normalized index that measures live greenness in an area and can be used as a proxy for vegetation. Hypothetically, NDVI values can range from -1.0 to 1.0. The IPUMS-DHS NDVI metric captures the maximum value of NDVI within the 10-kilometer buffer area (i.e., which pixel in the buffer has the maximum NDVI value for a given month), so the values in IPUMS-DHS for NDVI range from 0 to 1.
By adding NDVI to your data cart, you are in effect adding 72 separate variables for:
the 60 individual months prior to the survey start date, denoted by the number of months prior to the survey start date (e.g., NDVI_01 is the maximum NDVI in the month before the survey start date; NDVI_60 is the maximum NDVI in the 60th month before the survey start date)
the month of the survey start date (NDVI_00)
the 11 individual months following the month of the survey start date, denoted by the number of months following the month of the survey start date (e.g., NDVI_A01 is the maximum NDVI in the month following the month of the survey start date)
The source data for NDVI come fromÂ the Moderate Resolution Imaging Spectroradiometer (MODIS [URL omitted from DDI.])Â dataset, which covers the period February 2000 to October 2018. Only IPUMS-DHS samples whose data collection period overlapped with February 2000 or later are included in the NDVI variable (e.g., Bangladesh 2000). Note, however, that a sample fielded beginning in February 2000 would have non-missing NDVI values for the survey start month and the 11 following months, but would have only missing data values for the 60 months preceding the survey start date.
Researchers who need NDVI values for the full 5 years preceding the survey start--for example, to relate to health outcomes for children under age 5--should restrict their analysis to samples fielded from February 2005 forward.
NDVI, like the other IPUMS-DHS contextual variables, is calculated and based on sources external to the DHS data. For more information on IPUMS-DHS contextual variables and how they are constructed, refer to the documentation [URL omitted from DDI.] on these variables.