2017-03-03
1
• Part 1: The scientific method
• Part 2: A few specific reflection on ‘quantitative issues’
• Part 3: Sampling (sampling bias)
• Part 4: Blinding
Sampling
• Population: The entire aggregations of cases in which
a researcher is interested
• Accessible population: Aggregate of cases that
confirm to designated criteria and that are accessible
for a study
• Target population: Aggregated of cases about which
that a researcher would like to generalize
• Sample: subset of the target population
•  generalizability
What is sampling?
• Sampling is the process whereby a researcher
chooses the sample
• Of major importance
– Representativeness
– Richness of data
– Strength of research
Sampling
• Probability
• Non-probability
Probability sampling (random selection)
• Simple random sampling
• Stratified random sampling
• Multistage cluster sampling
• Systematic sampling
Probability sampling (random selection)
• Simple random sampling
– Establish random frame, random number table
• Stratified random sampling
• Multistage cluster sampling
• Systematic sampling
Probability sampling (random selection)
• Simple random sampling
• Stratified random sampling
– Subdivide population into homogeneous subsets
(e.g. gender, illness severity) from which an
appropriate number of elements are selected at
random
Probability sampling (random selection)
• Simple random sampling
• Stratified random sampling
• Multistage cluster sampling
Probability sampling (random selection)
• Simple random sampling
• Stratified random sampling
• Multistage cluster sampling
• Systematic sampling: selecting every k-th case from a
list
Sampling
• Probability
• Non-probability
2017-03-03
Non probability
• Convenience: most conveniently available
– Incl snowball , network
• Quota
• Consecutive
• Purposeful
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Non probability sampling
• Convenience
• Quota: researcher determines how may participants
are needed from each stratum
• Consecutive
• Purposeful
Non probability sampling
• Convenience
• Quota
• Consecutive: all the people who meet the criteria over
a specific time interval or for a specified sample size
• Purposeful
Non probability sampling
• Convenience
• Quota
• Consecutive
• Purposeful : judgmental sampling using the
knowledge of the researcher about the population to
select sample members
Sampling errors
• Introduce error due to selection of the sample
• Unavoidable?
2017-03-03
16
• Part 1: The scientific method
• Part 2: A few specific reflection on ‘quantitative issues’
• Part 3: Sampling (sampling bias)
• Part 4: Blinding

If done, who was blinded after assignment to
interventions (for example, participants, care
providers, those assessing outcomes) and
how
Example reporting on blinding in consort
1
Blinding (masking)
• Single blind: the individual subjects do not know
whether they are so-called "test" subjects or members
of an "experimental control" group. The person who
does the experiment knows group assignment
• Double blind: neither the participants nor the
experimenters know who is receiving a particular
treatment (correct for placebo effect)
Blinding
• participants (e.g. patients or healthy people);
• healthcare providers (e.g. the doctors or nurses
responsible for care);
• outcome assessors, including primary data collectors
(e.g. interview staff responsible for measurement or
collection of outcome data) and any secondary
assessors (e.g. external outcome adjudication
committees);
• data analysts (e.g. statisticians); and
• manuscript writers
Questions?