Correlational, or non-experimental, research is research where subjects are not acted upon, but where research questions can be answered merely by observing subjects.
An example of a correlational research question could be, "What is relationship between parents who make their children wash their hands at home and hand washing at school?" This is a question that I could answer without acting upon the students or their parents.
Quasi-Experimental Research is research where an independent variable is manipulated, but the subjects of a study are not randomly assigned to an action (or a lack of action).
An example of quasi-experimental research would be to ask "What is the effect of hand-washing posters in school bathrooms?" If researchers put posters in the same place in all of the bathrooms of a single high school and measured how often students washed their hands. The reason the study is quasi-experimental is because the students are not randomly selected to participate in the study, they just participate because their school is receiving the intervention (posters in the bathroom).
Experimental Research is research that randomly selects subjects to participate in a study that includes some kind of intervention, or action intended to have an effect on the participants.
An example of an experimental design would be randomly selecting all of the schools participating in the hand washing poster campaign. The schools would then randomly be assigned to either the poster-group or the control group, which would receive no posters in their bathroom. Having a control group allows researchers to compare the group of students who received an intervention to those who did not.
How to tell:
The only way to tell what kind of experimental design is in an article you're reading is to read the Methodologies section of the article. This section should describe if participants were selected, how they were selected, and how they were assigned to either a control or intervention group.
Random Selection means subjects are randomly selected to participate in a study that involves an intervention.
Random Assignment means subjects are randomly assigned to whether they will be in a control group or a group that receives an intervention.
Controlled Trials are trials or studies that include a "control" group. If you were researching whether hand-washing posters were effective in getting students to wash their hands, you would put the posters in all of the bathrooms of one high school and in none of the bathrooms in another high school with similar demographic make up. The high school without the posters would be the control group. The control group allows you to see just how effective or ineffective your intervention was when you compare data at the end of your study.
Randomized Controlled Trials (RCTs) are also sometimes called Randomized Clinical Trials. These are studies where the participants are not necessarily randomly selected, but they are sorted into either an intervention group or a control group randomly. So in the example above, the researchers might select had twenty high schools in South Texas that were relatively similar (demographic make up, household incomes, size, etc.) and randomly decide which schools received hand washing posters and which did not.
To tell if an article you're looking at is a Randomized Control Trial (RCT) is relatively simple.
First, check the article's publication information. Sometimes even before you open an article, you can tell if it's a Randomized Control Trial. Like in this example:
If you can't find the information in the article's publication information, the next step is to read the article's Abstract and Methodologies. In at least one of these sections, the researchers will state whether or not they used a control group in their study and whether or not the control and the intervention groups were assigned randomly.
The Methodologies section in particular should clearly explain how the participants were sorted into group. If the author states that participants were randomly assigned to groups, then that study is a Randomized Control Trial (RCT). If nothing about randomization is mentioned, it is safe to assume the article is not an RCT.
Below is an example of what to look for in an article's Methodologies section:
If you know when you begin your research that you're interested in just Randomized Control Trials (RCTs), you can tell the database to just show you results that include Randomized Control Trials (RCTs).
In CINAHL, you can do that by scrolling down on the homepage and checking the box next to "Randomized Control Trials"
If you keep scrolling, you'll get to a box that says "Publication Type." You can also scroll through those options and select "Randomized Control Trials."
If you're in PubMed, then enter your search terms and hit "Search." Then, when you're on the results page, click "Randomized Controlled Trial" under "Article types."
If you don't see a "Randomized Controlled Trial" option, click "Customize...," check the box next to "Randomized Controlled Trial," click the blue "show" button, and then click on "Randomized Controlled Trial" to make sure you've selected it.
This is a really helpful way to limit your search results to just the kinds of articles you're interested in, but you should always double check that an article is in fact about a Randomized Control Trial (RCT) by reading the article's Methodologies section thoroughly.