Page 335 - Contributed Paper Session (CPS) - Volume 4

CPS2253 Gabrielle Palermo
1. Introduction
Geres was designed to follow the imitial sampled pupils along five waves, from
2005 to 2008, and the new pupils that joined the classes of original pupils,
since they continued studying at a sampled school. The surveys additions
could be from the same initial population or a different population rather than
the one at wave 1. Additional pupils from the same population can have their
selection probability computed if the sampling frame is available. However, it
would be necessary to add all students studying with them in 2005 (1° grade),
and track them over the waves. As a consequence, the πhk would change for
(1)
all selected schools, and the sample representation for the study population
could fail.
On the other hand, some of the new students could be studying in a grade
above, and repeat the year at some point, joining a surveyed classes. Likewise,
an additional student could be studying in another place that is not part of
the area covered by the survey. Thus, pupils that are from a different
population at wave 1 do not have an inclusion probability.
Indirect sampling is crucial when one wants to study rare populations that
are difficult to identify or populations of interested that are not listed; since a
probability sample cannot be selected directly. The idea of indirect sampling
happens when a sample from a given target population B can be reached
through links with another sample of a population A, then, these two
populations are somehow connected. Indirect sampling is a form of
probability sampling because it is based on a sampling frame (Lavallée, 2007),
and it started to be used in cross-sectional surveys to make inference about
the population of interest.
The figure 1 shows the different ways that pupils could be included or
followed by Geres. The pupils that are outside school were not selected then.
And pupils could repeat or skip a grade more than once, but the figure only
shows pupils that were attending up to one year less or more than the main
grade, in other to make it simpler.
The main grade is the one initial pupils are expected to be studying at a
given wave. The first and the second waves happened at the same academic
year, and the further ones after one year. Thus, the main grade for each wave
is:
Wave 1: 1st grade;
Wave 2: 1st grade;
Wave 3: 2nd grade;
Wave 4: 3th grade;
Wave 5: 4th grade.

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