3.7 Experimental designs | Quantitative methods | Research Designs | UvA
Summary
TLDRThis script discusses the importance of true experiments for maximizing internal validity in research. It outlines key elements such as manipulation of the independent variable, comparison between conditions, and random assignment. The video introduces four common experimental designs: the two-group design, the two-group pre-test post-test design, the Solomon four-group design, and the repeated measures design. Each design is tailored to address potential threats to internal validity, such as maturation and pre-test sensitization, ensuring robust conclusions can be drawn from the study.
Takeaways
- 🔬 A true experiment is essential for maximizing internal validity in research.
- 🧬 Key elements of a true experiment include manipulation of an independent variable, comparison between different conditions, and random assignment to these conditions.
- 📝 Four common experimental designs are discussed: two-group, two-group pre-test post-test, Solomon four group, and repeated measures.
- 🎲 The two-group design involves random assignment to an experimental and control condition, with the independent variable differing between them.
- 📊 The two-group pre-test post-test design adds a pre-assessment of the dependent variable to account for initial differences and potential maturation effects.
- 🚫 The pre-test can sometimes alert participants to the study's purpose or cause a practice effect, impacting internal validity.
- 🤔 The Solomon four group design combines pre-test and no pre-test conditions to account for potential pre-test effects.
- 🔄 The repeated measures design involves participants experiencing all levels of the independent variable, with random assignment to the order of conditions.
- 📐 This design controls for individual differences by using the same subjects in all conditions, but can introduce order effects.
- 📈 The effectiveness of a pre-test can be assessed by comparing results from groups with and without it, informing the design of follow-up studies.
- 📚 The transcript emphasizes the importance of considering various threats to internal validity and the use of different designs to mitigate them.
Q & A
What is a true experiment and why is it important for maximizing internal validity?
-A true experiment is a research design that includes manipulation of the independent variable, comparison between conditions with different levels of the independent variable, and random assignment to these conditions. It is important for maximizing internal validity because it helps ensure that any observed effects are due to the independent variable rather than extraneous factors.
What are the key elements of a true experiment?
-The key elements of a true experiment are manipulation of the independent variable, comparison between conditions exposed to different levels of the independent variable, and random assignment of participants to these conditions.
What is the simplest experimental design mentioned in the script?
-The simplest experimental design mentioned is the two-group design, where participants are randomly assigned to an experimental condition and a control condition.
What is the purpose of the experimental condition in a two-group design?
-The purpose of the experimental condition is to introduce the hypothesized cause or independent variable to observe its effect on the dependent variable.
What is the purpose of the control condition in a two-group design?
-The control condition serves as a baseline to compare against the experimental condition, where the hypothesized cause is absent, allowing for the assessment of the independent variable's effect.
What is a two-group pre-test post-test design and why is it used?
-A two-group pre-test post-test design adds a pretest of the dependent variable before exposure to the independent variable. It is used to check for initial equality between groups and to control for maturation or other threats to internal validity.
What is a potential issue with using a pretest in an experiment?
-A pretest can sometimes sensitize participants, leading to a practice effect or alerting them to the study's purpose, which may negatively affect internal validity.
What is the Solomon four group design and how does it address the issues with pretests?
-The Solomon four group design is a combination of the two-group design and the two-group pre-test post-test design, run twice—once with a pretest and once without. It helps to account for the unwanted effects of pretests by comparing the results of groups with and without pretests.
What is the repeated measures design and how does it differ from between-subjects designs?
-The repeated measures design involves one within-subjects factor where all participants are exposed to all levels of the independent variable. Unlike between-subjects designs, random assignment is not to the conditions themselves but to the order in which the conditions are experienced.
Why might a researcher choose to use a repeated measures design?
-A researcher might choose a repeated measures design to control for individual differences between participants, as each participant serves as their own control by experiencing all conditions.
How does the order of conditions in a repeated measures design affect the study?
-The order of conditions in a repeated measures design can affect the results due to factors like learning effects, fatigue, or other order-related biases. Random assignment to the order helps to mitigate these effects.
Outlines
🔬 Maximizing Internal Validity in True Experiments
This paragraph discusses the importance of maximizing internal validity in true experiments through the manipulation of the independent variable, comparison between conditions, and random assignment. It introduces the two-group design as the simplest experimental design, where participants are randomly assigned to either an experimental or control condition. The paragraph also touches on the potential issues with randomization in small groups and introduces the two-group pre-test post-test design to account for maturation threats and to assess natural improvement versus the effect of the independent variable. It further explains the Solomon four-group design, which combines pre-test and post-test conditions to mitigate the effects of pre-test sensitization, and concludes with the repeated measures design, which involves participants experiencing all levels of the independent variable in a within-subjects factor setup.
Mindmap
Keywords
💡True Experiment
💡Internal Validity
💡Independent Variable
💡Dependent Variable
💡Random Assignment
💡Two Group Design
💡Pre-test Post-test Design
💡Solomon Four Group Design
💡Repeated Measures Design
💡Maturation
💡Practice Effect
Highlights
A true experiment is essential for maximizing internal validity in research.
Key elements of a true experiment include manipulation of the independent variable, comparison between different conditions, and random assignment.
Four common experimental designs are discussed for implementing these elements.
The simplest design is the two-group design with random assignment to experimental and control conditions.
The independent variable can differ in amount or kind between conditions.
The two-group pre-test post-test design is used to check initial proficiency and control for maturation effects.
A pretest can introduce practice effects or alert participants to the study's purpose, affecting internal validity.
The Solomon four-group design combines pre-test and post-test conditions to account for pretest effects.
This design helps to discern the effect of the independent variable despite practice effects.
A repeated measures design involves all participants experiencing all levels of the independent variable.
In repeated measures, random assignment is to the order of condition experience, not the conditions themselves.
The repeated measures design controls for individual differences by having the same subjects in all conditions.
A follow-up study can refine tests to better show differences between pre-test and non-pretest groups.
Experimental designs must consider threats to internal validity such as maturation and practice effects.
Random assignment is crucial for controlling pre-existing differences in the dependent variable.
The choice of experimental design depends on the research question and potential threats to validity.
Each design has its strengths and limitations in maximizing internal validity and controlling confounding variables.
Transcripts
a true experiment is the best way to
maximize internal validity the key
elements of a true experiment are
manipulation of the independent variable
comparison between conditions exposed to
different levels of the independent
variable and of course random assignment
to these
conditions of course these elements can
be implemented in very different ways
I'll discuss four experimental designs
that are very common the simplest design
is the two group design participants are
randomly assigned to one of two
conditions usually an experimental
condition where the hypothesized cause
is present and a control condition where
it's
absent the independent variable could
also differ between the conditions in
amount or kind for example if we're
investigating the effect of male versus
female Math teachers on math performance
of boys for example
in the two group design the dependent
variable is measured after exposure to
the independent variable to assess the
difference between the conditions which
are likely to be similar in all respects
due to the random
assignment including their pre-existing
position on the dependent
variable of course in small groups
randomization doesn't always work in
such cases it might be wise to use a two
group pre-test post test design which
adds a preest of the dependent variable
before exposure to the independent
variable with a pretest you can check
whether for example both groups of boys
were equally proficient in math before
being exposed to a female versus a male
math teacher for a
month this is an especially good idea
when maturation forms a plausible threat
to internal validity a pretest also
allows the researcher to compare the
size of the increase or decrease in
scores in the experimental and control
condition for example we can assess how
much the boy's math performance
increased due to Natural Improvement and
what the additional effect of teacher
sex
was unfortunately a preest can sometimes
sensitize participants the pretest may
result in a practice effect leading to
higher scores on the post test or it may
alert participants to the purpose of the
study especially if this effect is
stronger for one of the conditions
internal validity will be negatively
affected
but there's a way to take such unwanted
effects of a pretest into account by
using a Solomon four group Design This
is a combination of the two group design
and the two group pre-test postest
design the experimental and control
condition are run twice once with a
preest and once without for example it's
possible that the math test isn't very
hard to begin with and provides good
practice in those math skills that the
boys still lack on the post test the
boys in both conditions get perfect
scores obscuring any effect that teacher
sex might
have if we had two other groups of boys
that didn't take the pre-test we might
see the effect of teacher sex because
these groups have had less
practice of course if we find a
difference between these groups could
still be attributable to an already
existing difference in math proficiency
but together with the results of the
pre-test groups we could come up with a
better more difficult test showing
differences between the two preest
groups and two non- pretest groups in a
follow-up
study another very common design is the
repeated measures design with one within
subjects factor in this design all
participants are exposed to all levels
of the independent variable they
experience all conditions for example we
could randomly select half of the boys
to have a female math teacher for a
month and then a male teacher the
following month and the other half of
the boys would be taught by the male
teacher during the first month and the
female teacher during the second month
the only thing that's really different
to the previous between subjects designs
is that the random assignment of
participants is not to the conditions
themselves because they experience all
of them but to the order in which the
conditions are
experienced
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