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Basics of Scientific Research (459500)Topic 10
Research DesignsDr Jihad ABDALLAHSource
Research Methods Knowledge Basehttp//www.socialr
esearchmethods.net/

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Establishing a Cause-Effect Relationship

• Generally, there are three criteria that you must
  meet before you can say that you have evidence
  for a causal relationship
• 1. Temporal Precedence you have to be able to
  show that your cause happened before your effect.
  
• 2. Covariation of the Cause and Effect you have
  to show that you have some type of relationship ?
  if program then outcome, if not program then not
  outcome
• 3. No Plausible Alternative Explanations

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Research Design

• Research design can be thought of as the
  structure of research it provides the glue that
  holds the research project together.
• A design is used to structure the research, to
  show how all of the major parts of the research
  project (the samples or groups, measures,
  treatments or programs, and methods of
  assignment) work together to try to address the
  central research questions.

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What are the "elements" that a design includes?

• Observations or Measures These are symbolized by
  an 'O' in design notation. An O can refer to a
  single measure (e.g., a measure of body weight),
  a survey, or a whole battery of tests or measures
  given out on one occasion.
• If you need to distinguish among specific
  measures, you can use subscripts with the O, as
  in O1, O2, and so on.
• Treatments or Programs These are symbolized with
  an 'X' in design notations.
• Usually, a no-treatment (control or comparison
  group) has no symbol for the treatment (some
  researchers use X and X- to indicate the
  treatment and control respectively).

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• Groups Each group in a design is given its own
  line in the design structure. if the design
  notation has three lines, there are three groups
  in the design.
• Assignment to Group is designated by a letter at
  the beginning of each line (i.e., group) that
  describes how the group was assigned. The major
  types of assignment are
• R random assignment
• N nonequivalent groups
• C assignment by cutoff
• Time Time moves from left to right. Elements
  that are listed on the left occur before elements
  that are listed on the right.

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Random Selection Assignment

• Selection is how you draw the sample of people
  for your study from a population.
• Assignment is how you assign the sample that you
  draw to different groups or treatments in your
  study.
• Example Let's say you drew a random sample of 20
  lambs from a population of 1000 lambs. That is
  random sampling. Now, let's say you randomly
  assign 10 of these lambs as control group and 10
  to receive a treatment (diet, immunization
  program etc). That's random assignment.

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• Random selection is related to sampling.
  Therefore it is most related to external validity
  (generalizability of results).
• Random assignment is most related to design. When
  we randomly assign experimental units to
  treatments we have, by definition, an
  experimental design.
• Therefore, random assignment is most related to
  internal validity. After all, we randomly assign
  in order to help assure that our treatment groups
  are similar to each other (i.e., equivalent)
  prior to the treatment.

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Types of Designs
A randomized experiment generally is the
strongest of the three designs when your interest
is in establishing a cause-effect relationship. A
non-experiment is generally the weakest in this
respect.
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Two group designs
"nonequivalent" because in this design we do not
explicitly control the assignment and the groups
may be nonequivalent or not similar to each other
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The simplest form of non-experiment is a one-shot
survey design that consists of nothing but a
single observation O. This is probably one of the
most common forms of research and, for some
research questions -- especially descriptive ones
-- is clearly a strong design.
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Experimental Designs
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Two-Group Experimental Designs

• 1. The two-group posttest-only randomized
  experiment
• The simplest of all experimental designs. In
  design notation, it has two lines (one for each
  group) with an R at the beginning of each line to
  indicate that the groups were randomly assigned.
• One group gets the treatment or program (the X)
  and the other group is the comparison group and
  doesn't get the program

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2. Pretest-posttest randomized experimental
design. (Analysis of Covariance Design
ANCOVA)
The ANCOVA design is a noise-reducing
experimental design. It "adjusts" posttest scores
for variability on the covariate (pretest).
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Designs with three or more groups
1. Completely Randomized Design (CRD)

• R O
• R X1 O
• R X2 O
• . . .
• . . .
• R Xk O

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2. Randomized Block Designs

• They require that the researcher divide the
  sample into relatively homogeneous subgroups or
  blocks (analogous to "strata" in stratified
  sampling).
• Then, the experimental design you want to
  implement is implemented within each block or
  homogeneous subgroup.
• The key idea is that the variability within each
  block is less than the variability of the entire
  sample. Thus each estimate of the treatment
  effect within a block is more efficient than
  estimates across the entire sample.

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Factorial Experiments
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Parallel lines ? No interaction effects in this
case
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Change in scale
An interaction effect exists when differences on
one factor depend on the level you are on another
factor.
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Change in rank ? Strong interaction effect
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Quasi-experimental designs
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The Nonequivalent Groups Design

• Used frequently in social research. Structured
  like a pretest-posttest randomized experiment,
  but it lacks random assignment.
• We most often use intact groups that we think are
  similar as the treatment and control groups.
• In education, we might pick two comparable
  classrooms or schools. In community-based
  research, we might use two similar communities.
• We try to select groups that are as similar as
  possible so we can fairly compare the treated one
  with the comparison one. But we can never be sure
  the groups are comparable.

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It's unlikely that the two groups would be as
similar as they would if we assigned them
randomly. Because it's often likely that the
groups are not equivalent, this design was named
the nonequivalent groups design.
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Regression-discontinuity design (RD)

• In this design, participants are assigned to
  program or comparison groups solely on the basis
  of a cutoff score on a pre-program measure.
• In fact, the RD design does not require that the
  pre and post measures are the same

• C indicates that groups are assigned by means of
  a cutoff score,

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