Search costs, mobile phones, and agricultural investment: a telephone directory intervention in Tanzania

Last registered on November 30, 2022


Trial Information

General Information

Search costs, mobile phones, and agricultural investment: a telephone directory intervention in Tanzania
Initial registration date
November 18, 2022

Initial registration date is when the trial was registered.

It corresponds to when the registration was submitted to the Registry to be reviewed for publication.

First published
November 30, 2022, 2:33 PM EST

First published corresponds to when the trial was first made public on the Registry after being reviewed.


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Primary Investigator

Cornell University

Other Primary Investigator(s)

PI Affiliation
University of Chicago
PI Affiliation
Tufts University

Additional Trial Information

On going
Start date
End date
Secondary IDs
Prior work
This trial does not extend or rely on any prior RCTs.
While mobile phones are increasingly widespread in Sub Saharan Africa, there remains no accompanying service like the Yellow Pages that allows phone users to find new mobile numbers at low cost. This study will test the impacts of reducing the cost of search for information by providing farmers a directory with the contact information of businesses in the Kagera region of Northwestern Tanzania. The study will test the differential impacts of a digital phone-based versus a paper-based directory, and will test for heterogeneous effects based on baseline networks. The primary outcomes relate to phone usage, search behavior, investment, prices, income-generating activities, and various measures of extra-village contact and trade over space.
External Link(s)

Registration Citation

Aker, Jenny, Brian Dillon and Jessica Rudder. 2022. "Search costs, mobile phones, and agricultural investment: a telephone directory intervention in Tanzania ." AEA RCT Registry. November 30.
Sponsors & Partners

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Experimental Details


The primary intervention is a business directory with a description and contact information for approximately 10,000 businesses in 6 districts of the Kagera region. The directory takes two forms: a printed paper directory similar to a traditional print Yellow Pages, and a digital directory that is available via both USSD and smartphone app. Households are assigned to control, paper directory, or digital directory.
Intervention Start Date
Intervention End Date

Primary Outcomes

Primary Outcomes (end points)
Our key outcomes are at the household level and fall into the following categories:

1. Communication and search activities, including phone usage, SMS usage, mobile money usage, travel outside the village, communication with firms, number of crop markets/buyers searched, phone-mediated price search, transportation costs, travel time, and ordering goods for delivery.

2. Quantity and price of purchases of agricultural inputs (crop and livestock), inputs for non-farm enterprises, and other consumer goods and services represented in the directory (mechanics, tailors, food away from home).

3. Farm activities and investment, including crop choice, factor usage, acres cultivated, receipt of extension and advice, quantity produced, quantity sold, location of sales, prices received.

4. Non-farm enterprise choices and outcomes, including number and location of suppliers, goods and services offered, price search, and exit/entry.

5. Directory usage (for the treated), sharing of directories, specific firms contacted and information sought.
Primary Outcomes (explanation)

Secondary Outcomes

Secondary Outcomes (end points)
Spillover treatment effects within and across small communities
Secondary Outcomes (explanation)

Experimental Design

Experimental Design
This is a household-level randomized control trial involving a sample of approximately 3400 households in 120 villages.

Prior to launching the RCT we conducted a census of enterprises in a wide range of sectors, in 102 locations in 6 districts of the Kagera region, Tanzania. The information collected from these enterprises forms the basis of our primary intervention, a business directory. Villages were chosen for the census if they had a population of at least 4500 people in the most recent census.

To choose study villages, we first excluded central urban areas by dropping the fully urban district of Bukoba Municipal, and excluding any villages within 20km of the center of the three largest towns/cities in the study area. We then enrolled three types of villages: the 30 smallest villages from the enterprise census used to populate the directory (those are the smallest 30 villages in the study area that have populations of 4500 people or above); the 30 largest non-census villages (the 30 villages in the study area with less than 4500 people); and 60 small villages randomly selected from a pool of villages with a population below a certain threshold. We randomly assigned 30 of the 60 small villages to pure control.

Within the 90 villages not assigned to pure control, households will be randomly assigned to control, paper treatment, and digital treatment.

The experiment is designed for the following comparisons:

1. Paper treatment v. Control and Digital Treatment v. Control (household-level randomization)
2. Heterogeneous effects based on whether the village was included in the enterprise census, and therefore has local businesses represented in the directory (household-level randomization, leveraging the RD around village size threshold for inclusion in the census)
3. Heterogeneous effects based on village size (household-level randomization)
4. Spillovers to untreated households based on comparing households in pure control villages to control households in villages with some treated households (village-level randomization)

Experimental Design Details
Not available
Randomization Method
Randomization of households into treatment will be done upon reaching the end of the baseline survey, by the survey software. Treatment status will be unknown by the enumerator until they reach the final baseline module.
Randomization Unit
The primary unit of randomization is the household, except for the spillover analysis, for which the primary unit of randomization is the village.
Was the treatment clustered?

Experiment Characteristics

Sample size: planned number of clusters
For the spillover analysis, 30 villages (clusters) were assigned to pure control, and 30 were assigned to treatment. In the latter 30 villages, we randomly assign 16 households to control, 12 to paper treatment, and 15 to digital treatment. In the pure control villages, we randomly assign 16 households to control.

Otherwise, randomization is at the household level.
Sample size: planned number of observations
3400 households.
Sample size (or number of clusters) by treatment arms
30 small population villages with treatment: 1290 households
30 small population villages pure control: 480 households
30 villages just below population threshold for inclusion in the directory: 840 households
30 villages just above population threshold for inclusion in the directory: 840 households
Minimum detectable effect size for main outcomes (accounting for sample design and clustering)

Institutional Review Boards (IRBs)

IRB Name
Cornell University Institutional Review Board
IRB Approval Date
IRB Approval Number