When I first saw the headline, "Replacing Vacant Lots With Green Spaces Can Ease Depression In Urban Communities" I thought it was just another journalist putting a causal claim on a correlational study. So I was surprised to read this statement about the design:
[Researcher Eugenia South] and her colleagues wanted to see if the simple task of cleaning and greening these empty lots could have an impact on residents' mental health and well-being. So, they randomly selected 541 vacant lots [in the city of Philadelphia] and divided them into three groups.
They collaborated with the Pennsylvania Horticultural Society for the cleanup work.
The lots in one group were left untouched — this was the control group. The Pennsylvania Horticultural Society cleaned up the lots in a second group, removing the trash. And for a third group, they cleaned up the trash and existing vegetation, and planted new grass and trees. The researchers called this third set the "vacant lot greening" intervention.
Here's more:
The team surveyed residents living near the lots before and after their trial to assess their mental health and wellbeing. "We used a psychological distress scale that asked people how often they felt nervous, hopeless, depressed, restless, worthless and that everything was an effort," explains South.
The scale alone doesn't diagnose people with mental illness, but a score of 13 or higher suggests a higher prevalence of mental illness in the community, she says.
People living near the newly greened lots felt better. "We found a significant reduction in the amount of people who were feeling depressed," says South.
As one commentator noted:
Previous research has shown that green spaces are associated with better mental health, but this study is "innovative," says Rachel Morello-Frosch, a professor at the department of environmental science, policy and management at the University of California, Berkeley, who wasn't involved in the research.
"To my knowledge, this is the first intervention to test — like you would in a drug trial — by randomly allocating a treatment to see what you see," adds Morello-Frosch.
Questions
a) How do we know that this is an experiment and not a correlational study?
b) What were the Independent and Dependent variables?
c) What was the design: Posttest only? Pretest-posttest? Repeated measures? Or concurrent measures?
d) Sketch a graph of the results described above.
e) Ask at least one question each about this study's construct, internal, external, and statistical validities.
f) Because this article was published in the open-access journal JAMA Network Open anyone can read the paper. Take a look carefully at the tables in the paper (especially Table 2). How strong do the results seem to you? Are the differences between the conditions large? Do you see improvements on all the measured variables, or just on a few of them?
g) Why does the design of this study help support the causal claim that "Replacing vacant lots with green spaces can ease depression…."? (Apply the three causal criteria.)