The referral analysis allows one to compute the travelling times and/or the distances along the least-cost paths (i.e. path between two locations that minimizes the total travel duration for those travelling along it, see Ray and Ebener, 2008) between two groups of health centers. Such path is different from a straight line as it does take into account the landscape constraints together with the modes and speeds of travel of the population.
For For example, one would want to know the distance and travel time between each primary health care facility and the nearest referral hospital in a given Province, or make sure that the travel time between each Basic Emergency Obstetric Care facilities (BEmOC) and the nearest Comprehensive Emergency Obstetric Care facilities (CEmOC) is below 2 hours in a given country.
Starting Starting with version 5.3.2 of AccessMod, the scaling up Referral module has been improved to run in parallel mode, which means it can take advantage of all CPUs that are allocated to the AccessMod Virtual Machine. This speeds up the referral computation (see details in Appendix 5).
If you are running a referral analysis with a large number of facilities, it is therefore better to first set up your Virtual machine with a maximum of CPUs and memory (see Section 4.2).
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The sections in the second part of the analysis panel can then be filled as follow:
Facilities selection
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(5) The "Permute group" option allows one to improve computation time when more facilities are selected in the starting group than in the destination one. But if this option is selected, 'Limit the spatial analysis to the closest pair time-wise' is not possible. We will not use this option for the exercise.
(6) The option "Output the layer of the paths among selected facilities", if checked, will output a polyline shapefile with all least-cost paths among all the start-destination pairs of points. We will not use this option for the exercise.
(7) The "Enable parallelization" option, if checked, allows the referral analysis to be parallelized on all available CPUs/cores that your AccessMod instance has access to, if it is possible, and if it is worth doing it with the available memory. If you are using Virtual box, the numbers of available cores can be changed in the settings of VirtualBox (see section 4.2). Using this option can drastically decrease the overall computation time for this analysis. We will not use this option for the exercise.
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