Several news outlets picked up on a study about high school students' screentime and their grades. The Huffington Post headlined their story, "One Hour Of Extra Screen Time Drags Down Teenagers' Grades"
The summary of the study read like this:
Teenagers who spend an extra hour a day surfing the internet, watching TV or playing computer games risk performing two grades worse in exams than their peers who don't, according to research by British scientists.
An extra hour in front of the TV or online at age 14-and-a-half was linked with 9.3 fewer exam points at age 16 -- equivalent to two grades, for example from a B to a D. Two extra hours was linked to 18 fewer points.
a) Based only on this summary, what kind of study do you think this is--correlational or experimental? What are the two key variables in this study?
b) Since the study is correlational, you might be surprised that the headline was causal, saying that "screentime drags down grades." Apply the three criteria for causation to this claim, given what you know about the methodology. Does it support the claim?
Later in the article, the researcher explains the following about the study:
"We only measured [screen time at age 14.5], but this is likely to be a reliable snapshot of participants' usual behavior, so we can reasonably suggest that screen time may be damaging to a teenager's grades," said Kirsten Corder of Cambridge's Centre for Diet and Activity Research, who co-led the work.
c) In this quote, the researcher seems to be focusing on one of the three criteria for causation. Which one? Which one is she ignoring?
d) Challenge question: In the quote above, the researcher mentions "We only measured [screen time at age 14.5], but this is likely to be a reliable snapshot of participants' usual behavior". What concept is the researcher discussing here? Is this concept relevant to the causal claim the researchers and journalists wish to make?
e) Does this study appear to be a cross-lag panel design? Why or why not?
a) This study is most likely correlational, because it seems very likely that the two key variables in the study--amount of time spent on screens and grades earned--were measured.
b) Covariance: Yes, the study's results do suggest that more screen time is associated with lower grades.
Temporal precedence: Yes, the study measured screen time two years before grades.
Internal validity: It's not clear if any third variables were controlled for in a regression analysis. Even if they were, a regression analysis cannot control for all possible third variables, so this study cannot achieve internal validity. For example, it's possible that boys spend more time on screens and also get lower grades, so gender could be a third variable. Students with less parental support might both spend more time on screens and achieve lower grades, so parental support could be a third variable, too.
c) By emphasizing that screen time was measured at age 14.5, the authors seem to be emphasizing the study's temporal precedence, but not its internal validity.
d) The comment is referring to the test-retest reliability of the screen time measure. The researcher is saying that she assumes that screen time will be a stable trait, and that it would have good test-retest reliability. Of course, test-retest reliability is an empirical question--you have to collect data to find out if it exists for some measure. Assuming is not enough. In addition, the test-retest reliability of a measure is a consideration for construct validity. Having good construct validity is important for a study, but construct validity does not, by itself, help a study support a causal claim.
e) This is probably not a cross-lag panel design. In such a design, you need to measure the two key variables (screen time and grades) twice: once at Time 1 and again at Time 2. By saying that they only measured screen time once, the researcher is also revealing that this study is not a cross-lag panel design.