Experimental Design Flashcards
Experimental design is perhaps the single most important topic in MCAT P/S. Use this deck to master types of variables, reliability and validity, relevant statistical concepts, and more.
In an experiment, which variable is manipulated by researchers?
independent variable
The independent variable is the variable that is controlled or changed by the experimenters. The goal is to assess whether these manipulations impact the dependent variable.
In the graphed results of a typical experimental study, which variable is on the y-axis?
dependent variable
The dependent variable is the variable that changes in response to the independent variable. This variable is usually plotted on the y-axis, which helps visualize the magnitude of the changes.
In an experiment, which variable is measured?
dependent variable
The dependent variable is the variable that is measured by the experimenters. The goal is to assess how differences in the dependent variable correspond to the intentional changes made by the researchers to the independent variable.
In the graphed results of a typical experimental study, which variable is on the x-axis?
independent variable
The independent variable is the variable that is manipulated by the experimenters. Since this variable is typically varied in consistent increments, it makes sense to plot it on the x-axis.
A researcher in Singapore is studying the impact of PTSD on burnout rate in police officers. If these study results were graphed, what would be on the y-axis?
Burnout rate
Here, burnout rate is being measured, making it the dependent variable. The dependent variable is typically plotted on the y-axis.
A researcher in Singapore is studying the impact of PTSD on burnout rate in police officers. If these study results were graphed, what would be on the x-axis?
PTSD status
Here, PTSD status is something that the experimenters believe may influence the dependent variable (burnout rate). Therefore, PTSD status is the independent variable, which is typically plotted on the x-axis.
True or false.
Experiments can have only one independent and one dependent variable.
False
Experiments, including on the MCAT, can have multiple independent and/or multiple dependent variables. This is usually clear from the results, which (if in graph form, for example) might show multiple bars at each data point or have multiple x- or y-axes.
In an experiment, which type of variable can explain the relationship between the independent and dependent variables?
mediating variable
As its name implies, a mediating variable essentially โgoes betweenโ two other variables to explain their relationship. For instance, imagine that an increase in variable A leads to an increase in variable C (put symbolically, A โ C). If this is found to be true because an increase in A leads to an increase in B (A โ B), which then leads to an increase in C (A โ B โ C), then variable B is a mediating variable.
In an experiment, which type of variable can explain variations in the strength of the relationship between the independent and dependent variables?
moderating variable
This concept can be tricky! Just imagine that an increase in an independent variable corresponds to an increase in a dependent variable. Under certain circumstances, it might correspond to a large increase, but under others, it might only produce a small increase. These circumstances, which โmoderateโ the strength of the relationship, are moderating variables.
What is socioeconomic status (SES) called when it impacts both the independent and dependent variables in a study, even if it is not the focus of the study?
confounding variable
This relationship can cause researchers to erroneously believe that the independent and dependent variables are related, when in fact their results are due to the presence of the confounding variable.
True or false.
If a researcher is studying the impact of the provision of medical information on patient adherence to doctor advice, and gender is found to play a role, gender must be a confounding variable.
False
Itโs easy to assume that just because gender isnโt being studied (that is, it isnโt the independent or dependent variable), that it must be a confounding variable! However, that is not accurate; it could be a mediating or moderating variable instead.
A study is designed to assess the impact of dog ownership on stress. One post-bac student believes this study is flawed because it does not also assess the impact of cat ownership. Is the student correct?
no
This question relates to study scope. A study design does not need to include the assessment of every possible factor, including those outside its scope! Here, the study aims only to assess dog ownership; adding another independent variable of cat ownership would fall outside this scope.
Imagine that across the country, flights tend to be delayed more on Mondays than on any other day. However, in the southwest U.S., flights are delayed far more often on Mondays, while in the eastern U.S., flights are only delayed slightly more often. Here, region of the country is what kind of variable?
A moderating variable
In this example, the region of the country โmoderatesโ (or affects the strength of) the existing relationship between the day of the week and the likelihood of flight delays.
In a study of the influence of poverty on alcoholism, poverty is found to be associated with stress, which in turn is associated with increased rates of alcoholism. Here, what kind of variable is stress?
mediating variable
Here, stress explains the relationship between poverty and alcoholism, making it a mediating variable.
A Ph.D. student develops an experiment in which she measures the impact of eating breakfast on both cortisol levels and subject-reported alertness. In this study, whether a subject ate breakfast is what kind of variable?
independent variable
Since the Ph.D. student is evaluating the effects of eating breakfast, breakfast-eating status is the independent variable. It is likely that certain subjects are told to eat breakfast and other subjects to skip it.
A Ph.D. student develops an experiment in which she measures the impact of eating breakfast on both cortisol levels and subject-reported alertness. In this study, how many dependent variables are present?
two
Here, both cortisol levels and subject-reported alertness are being measured by the researcher in response to changes in the independent variable. Therefore, both are dependent variables.
True or false.
Age is a common confounding variable.
True
As age has extremely widespread effects, it often correlates with independent and dependent variables even when it is not being studied itself. Therefore, it is a classic confounding variable, and studies often use age-controlled/same-age subjects to control for it.
What is the term for causality in which variable A impacts variable B, but variable B has no effect on variable A?
unidirectional causation
This is common across the board in scientific research, to the extent that we usually assume relationships to be unidirectional unless indicated otherwise.
What is the term for causality in which two variables each impact the other?
reciprocal causation
While less common than unidirectional causation, this certainly still exists. For example, if researchers are evaluating the impact of depression on alcoholism, they may need to consider that alcoholism may also worsen some of the symptoms of depression.
What is the term for the group that is compared to the experimental group in nearly all scientific experiments?
control group
Virtually all well-designed studies include at least one control group, which is a group included for the purpose of comparison to the experimental group(s). Control groups allow the researchers to minimize the impact of confounding variables.
A control group that is expected to exhibit no change (in other words, that represents no effect) is what kind of control?
negative control
For example, a negative control group may simply not be treated with anything, while the experimental group receives the experimental treatment.
A control group that is expected to display a result based on existing understanding about the results in that group is what kind of control?
positive control
For example, if experimenters are testing whether a new treatment decreases levels of an enzyme, they may compare it with a control treatment that is already known to decrease levels of that enzyme.
Fill in the blank.
In human studies, researchers often go to great lengths to ensure that the negative control group believes that they may be receiving the experimental treatment. This best relates to the ________ effect.
placebo
The placebo effect refers to the impact of simply believing that one is receiving a treatment. As such, the negative control group is typically given a placebo treatment (such as a sugar pill) rather than being given nothing.
What does the epidemiologist ensure by carefully writing the Methods section of his paper so other researchers can understand and repeat his experiment?
replicability
It is crucial that experiments be replicable, meaning that other scientists can repeat the experimental method and steps and see if they get the same results.