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The Behavioral Effect of Facial Masks During the COVID-19 Pandemic
Last registered on June 05, 2020


Trial Information
General Information
The Behavioral Effect of Facial Masks During the COVID-19 Pandemic
Initial registration date
April 17, 2020
Last updated
June 05, 2020 9:26 AM EDT
Primary Investigator
Other Primary Investigator(s)
PI Affiliation
PI Affiliation
PI Affiliation
PI Affiliation
PI Affiliation
PI Affiliation
HU Berlin
Additional Trial Information
Start date
End date
Secondary IDs
The question of whether wearing face masks in public is an effective measure in slowing the spread of COVID-19 is subject of current public discussion. To add to this debate, we conduct a field experiment investigating whether face masks affect how individuals comply with public health recommendations such as keeping a physical distance of 150 cm. Using two treatments (with and without mask), experimenters wait in lines to enter businesses to measure the distance kept by the person next in line. We test whether wearing a face mask impacts the distance kept by others.
External Link(s)
Registration Citation
Balleyer, Anna et al. 2020. "The Behavioral Effect of Facial Masks During the COVID-19 Pandemic." AEA RCT Registry. June 05. https://doi.org/10.1257/rct.5735-2.0.
Experimental Details
We test whether a proximate person (subject) keeps a different distance to a proximate person (experimenter) depending on wearing a facial mask.
Intervention Start Date
Intervention End Date
Primary Outcomes
Primary Outcomes (end points)
The primary outcomes of the analysis will be the measured distance between the experimenter and the subject standing behind in the line and an indicator for whether or not the subject complies with the distance of 150cm.
Primary Outcomes (explanation)
Secondary Outcomes
Secondary Outcomes (end points)
Secondary Outcomes (explanation)
Experimental Design
Experimental Design
The experimenters position themselves at the end of a line to enter a business. As soon as an individual joins the line behind the experimenter, the experimenter records the distance to the next individual in line behind them. Shortly after the measurement, the experimenter leaves to rejoin the end of the line. The experimenter varies whether or not to wear an FFP2 mask according to a predefined protocol.
Experimental Design Details
The experimenters position themselves at the end of a line (e.g. in front of the post office, see experimental protocol for details). The experimenters are adjusting themselves facing away from the line such that the person in front of them stands to their one side and the arriving subject on their opposing side; this allows the experimenters to better register incoming subjects and turn toward them to conduct the subsequent distance measurement. Then, the experimenters calibrate a measurement app on their mobile device and await the subject’s arrival to start the measurement. After the measurement is taken, the experimenters immediately record the observed variables as described below on their mobile device. Experimenters vary whether or not they wear a facial mask according to the experimental protocol. As described in the experimental protocol, experimenters measure the key dependent variable of this study using an augmented-reality measuring app. Specifically, the distance in centimeters between the experimenter and the subject next in line at the moment of joining the line and standing still will be used to assess to what extent the subject adheres to the public safety recommendation of keeping a distance of at least 150 cm. Furthermore, visibly accessible data about the subjects is recorded to be used in the analysis. Specifically, experimenters record the subjects’ gender, age, whether the subject is wearing any kind of mask themselves, and whether they are accompanied. If subjects are accompanied, experimenters record how many individuals are joining the subject and whether this group includes children or only adults. Furthermore, the time and date, as well as the address and store of observation, are recorded by each experimenter.
Randomization Method
Before each session, each of the experimenters set an even target of observations to be recorded. Half of the observations should be executed with, and the other half without a mask on. The order is decided randomly using a fair coin or any random number generator by the experimenter before starting the respective session. Example: The experimenter sets the number to 20. After tossing the coin, the sessions start with 10 observations with the mask on. Then the experimenter removes the mask and performs another 10 without.
Randomization Unit
The treatment is randomized within experimental sessions as described in the randomization method.
Was the treatment clustered?
Experiment Characteristics
Sample size: planned number of clusters
The data will be collected by 5 experimenters, each collecting 60 observations. The number of clusters depends on the number of sessions onto which the experimenters will split their data collection. Details are specified in the experimental protocol.
Sample size: planned number of observations
The planned number of total observations is 300.
Sample size (or number of clusters) by treatment arms
150 with mask, 150 without mask
Minimum detectable effect size for main outcomes (accounting for sample design and clustering)
Overall 22 observations were collected in a pilot study, thereof 12 without and 10 with the face mask. The initial data showed that subjects kept a greater distance to the experimenter in the mask condition (M=152.5 cm, SD=52.35) compared to no mask condition (M=139.8 cm, SD=34.88), the difference, however, was not significant. Using the pilot data and the power command in Stata, we compute that in a sample with 300 observations, a two means t-test with an alpha of 0.5 and an effect similar in size to the one in the pilot data (raw difference of 12.7cm) and comparable standard deviations would have a power of 0.7. This corresponds to a minimum detectable effect size of about 14.4cm difference in the means for a two-sample t-Test with unequal variances at a power of at least 0.8.
Supporting Documents and Materials

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Request Information
IRB Name
IRB Approval Date
IRB Approval Number
Post Trial Information
Study Withdrawal
Is the intervention completed?
Intervention Completion Date
April 24, 2020, 12:00 AM +00:00
Is data collection complete?
Data Collection Completion Date
April 24, 2020, 12:00 AM +00:00
Final Sample Size: Number of Clusters (Unit of Randomization)
Was attrition correlated with treatment status?
Final Sample Size: Total Number of Observations
300 observations (i.e. subjects waiting in line after the experimenter)
Final Sample Size (or Number of Clusters) by Treatment Arms
150 observations (i.e. subjects waiting in line after the experimenter) in the treatment with mask and 150 observations in the treatment without mask
Data Publication
Data Publication
Is public data available?
Program Files
Program Files
Reports, Papers & Other Materials
Relevant Paper(s)
Governments across the world have implemented restrictive policies to slow the spread of COVID-19. Mandatory face mask use has been a controversially discussed policy, among others, due to potential adverse effects on physical distancing. Using a randomiz
Gyula Seres, Anna Balleyer, Nicola Cerutti, Anastasia Danilov, Jana Friedrichsen,Yiming Liu, MŸge SŸer. "Face Masks Increase Compliance with Physical Distancing Recommendations During the COVID-19 Pandemic,"ÊMay 23, 2020.