It seems intuitive （直觉的）that children's schoolwork will get worse if they spend too much time gazing at their phones instead of getting to bed or getting some exercise. And that is broadly what a published study has found. But is it the final word, and should parents be panicked into pulling the plug on their kids' devices?
Researchers in Canada analyzed lifestyle data from questionnaires taken by 4,520 US children aged 8 to 11. The children also performed a variety of standard cognition tests.
Jeremy Walsh at the Children's Hospital of Eastern Ontario Research Institute in Ottawa, Canada, and his colleagues evaluated how well the children met various Canadian government guidelines. These suggest limiting screen time to 2 hours a day, sleeping for 9 to 11 hours a night and spending at least an hour being physically active.
More than a third—1,655 children一met the guideline for limiting screen time, and their average performance in the cognitive tests was 4.5 per cent higher than that of the 1,330 children who met none of the guidelines. The gain was even higher, at 5.2 per cent, for those meeting both the screen-time and sleep recommendations.
So, armed with these results, should parents be clamping down on screen use? Walsh himself says the findings are temporary. "All these results need to be tempered by the fact that it was only a snapshot of children at one point in time," he says.
Another limitation is that the survey didn't reveal what the children were doing on their screens, which could be educational or trivial （无聊的）.“The study is limited by treating all screen time as equal,” says Heather Kirkorian at the University of Wisconsin-Madison. “To truly understand the impact of digital media on children, researchers must understand not only how much, but also how, what, where and with whom they’re watching.”
It is also unclear whether the questionnaires can produce accurate data. "The 8- to 11-year-old children reported their own screen and physical activity behavior, and many may have struggled to do this accurately," says Kirsten Corder at the University of Cambridge. "Data like these are likely to have different types of error which can make it harder to be certain about the results.”
However reliable the results, researchers agreed that parents should try to set some limits on screen time, especially when bedtime approaches. "Screen time before bed is doubly problematic because it keeps kids up later, and exposure to light impairs （损害）sleep quality,” says Eduardo Esteban Bustamante at the University of Illinois in Chicago.
Walsh has similar feelings: "We show that excessive screen time before bed has a negative impact on sleep, which is important for development of cognition and the brain generally" he says.
Why does Corder doubt the data from the questionnaires?
A、They may not be relevant.
B、They may not be valid.
C、They may not be up-to-date.
D、They may not be complete.
【正确答案】：B
