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It is true that correlation (or association between two variables) is not a sufficient reason for concluding that there is a cause-and-effect relationship between the correlated variables. I think the first two criteria are fairly straight-forward, but I will discuss the third in a little more detail a bit later.
So if correlational studies aren’t able to prove cause and effect, why do scientists use them? What enables scientists to talk about causal explanations, then, is to place less emphasis on the design of the study (though that is, of course, important) to show whether causal explanations are possible, and instead talk about alternative explanations. What this all means, then, is that in the end it is not so much the type of study design one uses (correlational, experimental, or a mix of both) that determines the strength of a study, but instead how well it rules out alternative explanations. Notes:Having said this, not being able to detect a correlation between two variables does not necessarily mean that the two are not causally related. Except where otherwise noted, content on this site is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported License. Spurious correlations have a ubiquitous effect in statistics.  Spurious correlations occur when two effects have clearly no causal relationship whatsoever in real life but can be statistically linked by correlation.
In the stork example, one cofounding factor is the size of the county – larger counties tend to have larger populations of women and storks and—as a clever replication of this study in the Netherlands showed—the cofounding factor is the weather nine months before the date of observation.  Partial correlation is the statistical test to identify and correct spurious correlations. We can also create the scatter plots for the interaction effects of our suspected control variable Aptitude Test 1.  The scatter plots show a medium, negative, linear correlation between our baseline test and the two tests in our analysis. It’s a statement which has become more well-recognized in the general public in recent years, which is a great trend. The point here is that correlation is an essential part of establishing the existence of cause and effect. Well, scientists have a broad range of research designs available to them to answer questions about the world.
If you remember the three criteria above, the third criterion is a lack of plausible alternative explanations. Before a particular study even gets to the point of being critiqued by others, the scientists conducting it take great care (ideally) to create a study that rules out as many alternative explanations as possible.
A well-designed correlational study that measures the right variables in the right way can be a very convincing study indeed. Correlation may not tell you everything about cause-and-effect relationships, but it can be a good indicator that something might be there. Sometimes, there can be other variables that suppress the relationship between a cause and effect, making it appear that they are unrelated. If an outcome happens when a factor is present, and does not happen when the factor is absent, there is a correlation.
Pearson's Bivariate Correlation Coefficient shows a positive and significant relationship between the number of births and the number of storks in a sample of 52 US counties. However, I still find it problematic because it is sometimes used to dismiss correlational research as somehow invalid or subpar.


In other words, you can have correlated variables that are not causally related, but you can’t have causally related variables that are not correlated!
This is why experimental designs are so preferable, because they are often much better at ruling out many alternative explanations for the results. It’s a valuable tool of science, and when used properly it can tell us much about what variables may cause other variables to change.
For example, gravity is causally acting on airplanes all the time, yet they are able to fly through the air because other forces counteract the force of gravity. Or, if an outcome increases or decreases when a factor increases or decreases, there is a correlation. For example, it is probably the case that the number of people wearing bathing suits is correlated with the number of shark attacks in a given year.
A variable that is presumed to be the cause might be impossible or impractical to manipulate directly (it is hard to make a star go supernova to study its effects, for example), or unethical (when studying the effects of the death of a spouse on people’s well-being, an ethics review board would probably frown on a study design that involved randomly assigning spouses to be killed). If I have truly changed only one thing between two otherwise identical groups, and I then find that those two groups differ on some important variable, I have extremely good evidence of cause and effect. Understanding the role of plausible alternative explanations is the key to understanding how science works. By ruling out alternative explanations, we can find our way toward a better understanding of how the world works. We say there is a correlation between the factor and the outcome.Graph showing number of cigarettes smoked against the number of deaths from cancerFor example, studies show that there is a correlation between how much a person smokes and how likely they are to get lung cancer. So I’d like to offer a more nuanced understanding of how to evaluate research, at a level that an educated but non-scientific audience can understand and appreciate.
However, this does not mean that wearing bathing suits causes shark attacks, or that shark attacks cause people to wear bathing suits. An experimental design requires two things: manipulating a variable (the presumed cause), and randomly assigning observations to either a treatment or control group. It is also often the case that a scientist has one primary variable they wish to manipulate, but they also want to take a look at the influence of other variables that they then measure. Correlational designs, while generally not as good at ruling out alternatives, can still offer  relatively strong evidence if done properly. Thus, it is most accurate to say that detecting a correlation is a necessary condition of inferring causality in the absence of countervailing forces. As the number of cigarettes smoked increases, the percentage of people who smoke that number and get lung cancer also increases. We can easily come up with a plausible reason: Summer weather causes both a greater number of people wearing bathing suits and a greater number of people in the water to be attacked by sharks.
In the treatment group, the variable is changed in some way, whereas in the control group it is left alone.3 For example, if I am running a psychological study on whether ice cream makes people happy, I might randomly assign participants to either be given ice cream (experimental group) or nothing at all (control group), and then ask them how happy they are on a scale from 1 to 10. For instance, I can manipulate whether people receive positive or negative feedback, but I can’t manipulate (for practical and ethical reasons) their self-esteem.


More advanced correlational techniques like cross-lagged designs can help to establish temporal precedence (criterion #2) by showing that the link between variable A at time 1 and variable B at time 2 is stronger than the link between variable B at time 1 and variable A at time 2.
However, if summer weather does indeed cause more people to wear bathing suits, those two variables should be correlated with each other.
This type of design has a number of advantages over a correlational design, but the one that is most relevant is the random assignment. If I want to study how people with low self-esteem react to negative feedback, then, I need to use a mixed design.
And of course, depending on the variables one is studying, it can be relatively easy to rule out some alternative explanations. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. By randomly assigning participants to the two groups, I can make sure that any differences between the two groups that are not related to the ice cream variable are due to chance. This all makes the scientific process very complicated, since there is a mix of manipulated and measured variables that each offer more or less control.
For example, one study has shown that violent crime rates increase as temperature rises (leading to more crime in summer months).4 While there could perhaps be other variables that affect both temperature and crime rates, one explanation that does not seem plausible is that crime itself actually increases the outside temperature. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so. This offers scientists a great deal of control, to ensure that the presumed cause is the only thing being changed between the two groups. The nature of the variables being studied can make some explanations wildly implausible, leading to a stronger case for a certain interpretation. If I were to simply ask people how much ice cream they ate in the past day and then measure their happiness, I don’t have that level of control. Perhaps people who eat more ice cream also eat more brownies, and it’s actually brownies that lead to greater happiness!
For those interested in learning more about the debate, the book has a good overview of the major philosophical traditions and their historical development.
You can also check out Bradford Hill’s criteria for causation, which list more criteria, but I think agree reasonably well with Cook and Campbell.
For instance, some research might compare one treatment to another, where the purpose is just to find out whether treatment A works better than existing treatment B. In a placebo design, the only difference (ideally) is the active ingredient of the drug being tested.
Using the placebo group holds constant other factors, like the act of taking a pill and the psychological influence involved in taking medication.



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