Since case studies only involve one situation, they may lead to the over-generalization of a condition that is rare and/or specific to a certain situation. 

This example is a double blind study. In a double blind study, both the subjects and researchers do not know which individuals are given the placebo and which are given the actual treatment. This type of study can control for the placebo effect and researcher bias. 

The population would be all the college students that have taken standardized tests in the country the psychologist resides in because a population is all the possible individuals in a group that a sample can be pulled from. It is most likely not possible that the researcher would be able to include the entire population in his study due to the large size.

When researchers assign study participants to different groups based on chance, such as by using a random number generator or flipping a coin as her, this is an example of random assignment. A method that is not random assignment would be something like assigning all men to one group and all women to another. Random assignment is sometimes confused with random selection, which means choosing study participants at random from the population being studied.

Experimental research designs allow researchers to carefully control each variable measured. This level of control allows researchers to establish causal connections between variables and allows other researchers to know exactly how to repeat the study on their own; however, this control also confines the study to variables that can be observed in a laboratory setting. Unlike descriptive studies, in which participants might not always know they are being observed, experiments force researchers to brief their participants on how they will participate, and allow participants to decline or leave.

We know that Dr. Andrews used a longitudinal design because the question tells us that he tracks the same group of children’s progress at regular intervals. This is the definition of a longitudinal study design: the same group of individuals is observed, measured, tested, etc. on the same dependent variable at regular time intervals. This allows the researcher to see how a certain measured trait changes in individuals over time; however, longitudinal studies take a long time to carry out and are quite expensive. As a result, many researchers use an alternative cross-sectional design, which uses two or more different groups of subjects that are relatively similar except in age and measures them just once. 

While all of these are things that a good experiment should possess, the one that is most important for a study’s ability to generate conclusions about causal relationships is the presence of a true independent variable. The fact that the researcher manipulates this variable directly allows him or her to conclude that any changes in the dependent variable are related to the changes in the manipulated independent variable alone. Causal relationships could still be inferred from a study that does not have a consent form or proper confidentiality practices; however, this study would certainly be unethical. Even research studies that do not focus on causal relationships involve the measurement of dependent variables.

Random assignment is the only term that is incorrectly matched. Random assignment involves assignment to experimental and control groups. It ensures that each participant has an equal chance of being assigned to a group. On the other hand, random selection refers to each member of a group having an equal chance of being selected for the study. 

Cross-sectional and longitudinal studies are both types of correlational studies that allow researchers to predict or draw conclusions from data. "Cross-sectional" studies allow researchers to examine how a sample differs on a specific characteristic at one point in time. "Longitudinal studies" involve following a sample over time. On the other hand, "experimental studies" include random selection, random assignment, and an independent variable that is manipulated. Causation can only be concluded from true experiments. In "double-blind experiments," both the researchers and participants are unaware of which groups are assigned as the experimental and control groups. Last, a "meta-analysis" allows researchers to look at several research studies and draw statistical conclusions. 

The independent variable is the specific group that participants are assigned to (i.e. drug vs. sugar pill group). The experimental group is the group that receives the intervention (i.e. new depression drug), while the control group is the group that receives no treatment or a placebo. The dependent variable in a study is what is being measured, which in the case of this study is depression scores. The term random assignment refers to each participant having an equal chance of being assigned to the experimental and control groups.