Untangling the effects of TV watching on mortality

Interpreting the results of studies can be difficult, particularly if one confuses a correlation (indicating some relationship between two variables) and a direct causal relationship (where one variable causes another). This usually is translated into the common phrase “correlation, not causation” which is illustrated in this example from Entertainment Weekly:

Researchers in Australia are reporting that, on average, every hour spent watching television after the age of 25 decreases the amount you live by 22 minutes.

“As a rule, the more time we spend watching TV, the more time we spend eating mindlessly in front of the TV, and the less time we spend being physically active,” explained Dr. David L. Katz, director of the Prevention Research Center at Yale University School of Medicine to HealthDay.com. “More eating and less physical activity, in turn, mean greater risk for obesity, and the chronic diseases it tends to anticipate, notably diabetes, heart disease and cancer.”

Before you throw your soul-sucking flat screen out the window, here’s a key thing to remember:

TVs are not like the year-draining torture machine in The Princess Bride. This study measures a casual lifestyle correlation — people who watch a lot of TV, on average, die younger than those who do not.

This seems to make sense – it is not TV watching that is the real issue but rather sitting around a lot, which is related to TV watching. This was echoed in the HealthDay story the EW post refers to:

But other experts cautioned that the study did not show that TV watching caused people to die sooner, only that there was an association between watching lots of TV and a shorter lifespan.

But I wonder if this is more of a conceptual issue that an analysis issue on the part of the original researchers. While I can’t access the original article, here is part of the abstract that sheds light on the issue:

Methods The authors constructed a life table model that incorporates a previously reported mortality risk associated with TV time. Data were from the Australian Bureau of Statistics and the Australian Diabetes, Obesity and Lifestyle Study, a national population-based observational survey that started in 1999–2000. The authors modelled impacts of changes in population average TV viewing time on life expectancy at birth.

Results The amount of TV viewed in Australia in 2008 reduced life expectancy at birth by 1.8 years (95% uncertainty interval (UI): 8.4 days to 3.7 years) for men and 1.5 years (95% UI: 6.8 days to 3.1 years) for women. Compared with persons who watch no TV, those who spend a lifetime average of 6 h/day watching TV can expect to live 4.8 years (95% UI: 11 days to 10.4 years) less. On average, every single hour of TV viewed after the age of 25 reduces the viewer’s life expectancy by 21.8 (95% UI: 0.3–44.7) min. This study is limited by the low precision with which the relationship between TV viewing time and mortality is currently known.

Conclusions TV viewing time may be associated with a loss of life that is comparable to other major chronic disease risk factors such as physical inactivity and obesity.

Some key parts of this:

1. This was done using life table models, not correlations. Without seeing the full article, it is hard to know exactly what the researchers did. Did they simply calculate a life table (see an example in 7.2 here) or did they run a model that included other independent variables?

2. Their confidence intervals are really wide. For example, the amount of TV watched in 2008 could only shorten someone’s life by 8.7 days, hardly a substantively significant amount over the course of a lifetime. Watching 6 hours a day on average (compared to those who watch no TV), could live just 11 minute shorter lives.

3. The abstract suggests there is “low precision” because this link hasn’t been studied before. If this is true, then we need a lot more science on the topic and more data. This article, then, becomes an opening or early study on the topic and is not the “definitive” study.

4. The conclusion section says “may be associated with a loss of life that is comparable to other major chronic disease risk factors such as physical inactivity and obesity.” The key word here is “may.” This might simply be an academic qualification but it is an important distinction between saying “proved” (how the public might want to interpret it).

Here is my guess at what happened: media reports (or perhaps even a press release) about the study were a lot more strident about these results than the researchers themselves. In fact, here is a piece from the HealthDay piece that suggests this may be the case:

Researchers in Australia found that people who averaged six hours a day of TV lived, on average, nearly five years less than people who watched no TV.

The emphasis here is on the average, not necessarily the confidence interval. This would be like reporting poll results that say a candidate leads by 6 over an opponent but forgetting to mention that the margin of error (a confidence interval) is 5.9.

What the HealthDay report should include: comments from the researchers themselves explaining the work. Interestingly, the story quickly suggests that other researchers say there are other factors at work but we never hear from the original researchers outside of a few pieces lifted from the study. Without the proper context, a study can become a “shock headline” used by media sites to drive traffic.

I do have to ask: does Entertainment Weekly have a vested interest in debunking a study like this since they are in the business of reviewing television shows and channels?

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