Social network targeting to maximise population behaviour change: a cluster randomised controlled trial

Abstract

Background: Information and behaviour can spread through interpersonal ties. By targeting influential individuals, health interventions that harness the distributive properties of social networks could be made more effective and efficient than those that do not. Our aim was to assess which targeting methods produce the greatest cascades or spillover effects and hence maximise population-level behaviour change.

Methods: In this cluster randomised trial, participants were recruited from villages of the Department of Lempira, Honduras. We blocked villages on the basis of network size, socioeconomic status, and baseline rates of water purification, for delivery of two public health interventions: chlorine for water purification and multivitamins for micronutrient deficiencies. We then randomised villages, separately for each intervention, to one of three targeting methods, introducing the interventions to 5% samples composed of either: randomly selected villagers (n=9 villages for each intervention); villagers with the most social ties (n=9); or nominated friends of random villagers (n=9; the last strategy exploiting the so-called friendship paradox of social networks). Participants and data collectors were not aware of the targeting methods. Primary endpoints were the proportions of available products redeemed by the entire population under each targeting method. This trial is registered with ClinicalTrials.gov, number NCT01672580.

Findings: Between Aug 4, and Aug 14, 2012, 32 villages in rural Honduras (25-541 participants each; total study population of 5773) received public health interventions. For each intervention, nine villages (each with 1-20 initial target individuals) were randomised, using a blocked design, to each of the three targeting methods. In nomination-targeted villages, 951 (74·3%) of 1280 available multivitamin tickets were redeemed compared with 940 (66·2%) of 1420 in randomly targeted villages and 744 (61·0%) of 1220 in indegree-targeted villages. All pairwise differences in redemption rates were significant (p<0·01) after correction for multiple comparisons. Targeting nominated friends increased adoption of the nutritional intervention by 12·2% compared with random targeting (95% CI 6·9-17·9). Targeting the most highly connected individuals, by contrast, produced no greater adoption of either intervention, compared with random targeting.

Interpretation: Introduction of a health intervention to the nominated friends of random individuals can enhance that intervention's diffusion by exploiting intrinsic properties of human social networks. This method has the additional advantage of scalability because it can be implemented without mapping the network. Deployment of certain types of health interventions via network targeting, without increasing the number of individuals targeted or the resources used, could enhance the adoption and efficiency of those interventions, thereby improving population health.

Funding: National Institutes of Health, The Bill & Melinda Gates Foundation, Star Family Foundation, and the Canadian Institutes of Health Research.

Conflict of interest statement

DECLARATION OF INTERESTS

ICMJE Conflict of Interest Statements for each author are submitted separately.

Copyright © 2015 Elsevier Ltd. All rights reserved.

Figures

FIGURE 1. Targeting of interventions for one block of villages

Within each block of four villages, we randomly assigned each village to a targeting method (indegree, nominated, random, or none). We did this for each of the two interventions in a fractional factorial design (see SI). In the indegree-targeted villages, we targeted the 5% of villagers with highest indegree. In the nomination-targeted villages, we targeted one randomly chosen friend nominated by each member of a 5% random sample of villagers. In the randomly targeted villages, we targeted a 5% random sample of villagers. In the villages receiving both interventions (chlorine and vitamins), a small proportion of villagers were drawn, by chance, as targets for both interventions (e.g., two blue nodes in Village 1). In the block shown, Village 1 received multivitamins (red nodes) by random targeting and chlorine (green nodes) by nomination targeting. Village 6 received multivitamins by nomination targeting and chlorine by indegree targeting. Village 25 received multivitamins only, by indegree targeting. Village 26, finally, received multivitamins by indegree targeting and chlorine by random targeting. Some visible groups of siblings are not fully inter-connected given a deliberate feature of the “name generator” used to map the network (see SI).

FIGURE 1. Targeting of interventions for one block of villages

Within each block of four villages, we randomly assigned each village to a targeting method (indegree, nominated, random, or none). We did this for each of the two interventions in a fractional factorial design (see SI). In the indegree-targeted villages, we targeted the 5% of villagers with highest indegree. In the nomination-targeted villages, we targeted one randomly chosen friend nominated by each member of a 5% random sample of villagers. In the randomly targeted villages, we targeted a 5% random sample of villagers. In the villages receiving both interventions (chlorine and vitamins), a small proportion of villagers were drawn, by chance, as targets for both interventions (e.g., two blue nodes in Village 1). In the block shown, Village 1 received multivitamins (red nodes) by random targeting and chlorine (green nodes) by nomination targeting. Village 6 received multivitamins by nomination targeting and chlorine by indegree targeting. Village 25 received multivitamins only, by indegree targeting. Village 26, finally, received multivitamins by indegree targeting and chlorine by random targeting. Some visible groups of siblings are not fully inter-connected given a deliberate feature of the “name generator” used to map the network (see SI).

FIGURE 2. Diffusion of interventions

The left panel shows the pooled proportion of available multivitamin tickets redeemed by day after initial targeting, by treatment arm. The right panel shows the equivalent measure for the chlorine intervention.

FIGURE 3. Multivitamin diffusion through two representative villages

In all villages receiving a given intervention, 5% of the adult population was initially targeted with the intervention. The left panel shows the diffusion of multivitamins in a village whose five initial targets were selected at random. The right panel represents a village of comparable size whose six initial targets were the nominated friends of random villagers. Every targeted villager received four tickets to distribute to contacts outside the household, who could in turn redeem the ticket for the same product and for four additional tickets to give to others. The colored lines represent completed ticket-redemption paths: shorter lines represent tickets redeemed more quickly. Light gray lines represent available tickets that were not redeemed. Compared to the randomly targeted village, multivitamins in the nomination-targeted village diffused more rapidly (the average time-to-redemption of first-wave tickets is substantially shorter) and more completely (a greater proportion of available tickets was ultimately redeemed).