Arai et al. (2020) : The Hidden Potential of Call Detail Records in The Gambia.

# **4. Data and methodologies**

# **A. CDR data descriptive**

The mobile penetration rate<sup>2</sup> of The Gambia was 94.2 percent in 2013 and rose to 140 percent in 2018, which was higher than the average of developed countries (“ITU ICT-Eye,” n.d.). As of 2018, 98.4 percent of households reported ownership of at least one mobile phone with limited variation across regions and between rural and urban areas. On the individual level, 85.1 percent of men and 74.1 percent of women in the age group between 15 and 49 years own a mobile phone (The Gambia Bureau of Statistics, 2019). According to PURA the four major MNOs provide services to 2.59 million subscribers as of 2020, even allowing for persons with multiple sim-cards. In this paper, we use CDR data for a three-month period between March 2, and May 31, 2020, offering a snapshot of changes in mobility during the COVID-19 lockdown.

Data was made available for two major MNOs, which cover approximately 70 percent of the market and include around 1.75 million subscribers. On average, the data comprises 18.8 million data points per day with very limited variations over the data period, 2 billion anonymized observations in total. Hence, we assume that cell phone usage in terms of transaction volumes did not change fundamentally once COVID-19 hit the country. The average number of records per subscriber per day is 10.6 where approximately 2.6 records are used for calling. Like other developing countries, multiple-SIM-card holding is common in The Gambia. We expect a certain overlap between the two-MNO subscribers, which might have resulted in over-representing the multiple-SIM-card holders. In this study the impact of the multi-SIM holding on the analysis result is considered to be limited since the two MNOs primarily market different socio-economic groups. One of them is a leading MNO in The Gambia and is popular in urban areas with high-speed internet services. The other MNO provides only voice and shortmessaging services with inexpensive plans, which are much popular in rural areas.

The preparation and analysis of CDR data under this project are based on a protocol to address concerns of privacy and confidentiality. Raw CDR data include several identifiers associated with each record, such as phone number, IMSI and IMEI. We employ a three-stage approach to anonymize these identifiers in order to protect data privacy.

- First, identifiers are encrypted using a one-way function by the MNOs on their premises to protect the data privacy.

- Second, lists of cell towers and the locations provided by the MNOs are pooled in the regulator’s premise and cell-tower locations are clustered using Ward’s hierarchical clustering, with a maximum distance constraint of 1km from the centroid of the cluster.<sup>3</sup> We then use the centroid of the cluster to match and map the de-identified CDR data to their respective celltower locations. This process lowers the spatial granularity of cell-tower distributions.

- Third, the results of all indicators are aggregated at the administrative unit level. There are certain concerns about reverse engineering for the re-identification of de-identified CDR data (Daniel Kondor, Hashemian, de Montjoye, & Ratti, 2018) but the abovementioned aggregation process lowers the risk of reverse engineering.

> 2 The mobile phone penetration rate is computed as the number of SIM cards, which is the number of connections to the mobile network services, divided by the population number.

> 3 The choices of a clustering methodology and a distance threshold are proposed by the World Bank COVID19 Mobility Task Force.

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