Statistical DesignSample DesignThe sample design EMIF Norte and EMIF Sur have the objective of estimating the size of migration flows and the characteristics of the individuals who make up those flows. A measurement of flow is defined as the total number of humans moving through a geographical space over a determined period of time. The period of time considered for this survey is a quarter of a year. After using a multistage sampling for 20 years to obtain these estimates (consult here: www.colef.mx/emif/publicacionesnte.php), it was decided to restructure the sample design and change it to a stratified sample design. This change took place as part of a joint evaluation by a working group of experts from the Center for Mathematical Investigation (CIMAT) and the participants of the survey projects. Among the arguments the working group gave to explain the sample design change, what most stands out is that the previous design had sampling stages in which the probability of selection was one, while other sampling stages had a proportional allocation procedure. Thus was proposed that these levels should be incorporated as part of a stratification, which resulted in a significant simplification of the sample design. A brief description of the new stratified sample design is given below. In the making of the sampling framework, two axes are created: time and space. The time axis is defined as the number of calendar days in the trimester, which can be 90, 91, or 92 (unless it is known that on certain days migration does not occur). Each one of the days is then divided into 1, 2, or 3 shifts, depending on the quantity of the migration flow and its distribution over the day's 24hour period. The second axis is composed of a list of survey points located in border cities where migrants transit. This list of survey points is the product of exhaustive fieldwork in which the entire border region is visited, entry points are identified and the total immigration flow is measured over the course of a seven day period 24 hours a day. The synthesis of the time and space axes defines the sampling framework of the study (see Table 1). Each of the cells in the table represents a combination of time and space, which we name a jornada. In terms of the sampling framework, each jornada is a sample point. Table 1. Sampling Framework
Note: S=shift

Flow of migrants  Number of subgroups  Total number of tours (1) 
Number of tours in the sample  Number of individuals contacted (2) 
Average completed surveys by trimester (3) 


Emif Norte 

Coming from the North  98  8,820  368  18,228  1,961  
Coming from the South  86  7,740  377  16,606  2,142  
Returned by U.S. Immigration Officials  20  1,748  153  1,046  1,046  
Coming from the United States by plane  13  1,170  300  21,844  2,604  
Emif Sur 

Coming from Guatemala  18  1,620  145  13,875  2,689  
Coming from Mexico or the United States  14  1,260  188  20,700  3,899  
Returned by Mexican Authorities  4  312  142  2,122  2,122  
Returned by U.S. Authorities  3  169  78  1,606  1,606 
For the selection of tours, first the sample size is distributed among the subgroups, assigning two tours to each subgroup. The rest are distributed proportionally according to the migrant flow distribution among the subgroups. Later, inside each subgroup the days of the quarter in which the fieldwork will be performed is chosen randomly and with equal probability. The number of days selected is equal to the sample size assigned to the subgroup.
The second stage of the sampling occurs within a tour, on the individual level. The selection is random and is defined by the moment at which the individual arrives at the survey point. To better understand this process it is helpful to mention that the survey fieldwork is designed so that two researchers collaborate during the selected tours. Each one of them carries out one of two activities: A) count B) screen individuals and apply the questionnaire.
Task A: To count. It is very important for the Mexican Migration Survey methodology to record the total number of people that compose a migration flow during a certain tour, since this is an essential input to later estimate the total number of migrants. Thus, a continuous count is carried out: A researcher acting as an enumerator places oneself in an advantageous position to constantly observe the flow of people. The researcher then imagines a straight line crossing the ground, and counts the number of people who cross that line.
Task B: To interview. The other essential piece of fieldwork is the application of the survey, which is generally divided into two sections. The first section is the socalled screening process, a series of eight or nine brief questions given by the surveytaker to a randomlyselected individual to see if that person is part of the target population or not. In the case of identifying a migrant, the surveytaker continues to the second section, which is a more extensive questionnaire whose questions vary depending on the migration flow being analyzed (see Diagram 1).
The implementation of these tasks may vary depending on the circumstances of each migration flow. For example, in the flow of Mexican migrants being repatriated by U.S. authorities, or Central Americans being repatriated by Mexican authorities, the screening questions are not given since all the individuals are known to be study subjects. At other locations and crossing points, conditions may vary so that modifications of tasks A and B become necessary.