The bane of many causes

One of the newsflies buzzing around today is an article “Brain tumour risk in relation to mobile telephone use: results of the INTERPHONE international case-control study”.

The results, shown in this pretty table below, appear to be inconclusive.

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A limited amount of cellphone radiation is good for your brain, but not too much? It’s unfortunate that the extremes are truncated. The commentary at Microwave News blames bias:

The problem with selection bias –also called participation bias– became apparent after the brain tumor risks observed throughout the study were so low as to defy reason. If they reflect reality, they would indicate that cell phones confer immediate protection against tumors. All sides agree that this is extremely unlikely. Further analysis pointed to unanticipated differences between the cases (those with brain tumors) and the controls (the reference group).

The second problem concerns how accurately study participants could recall the amount of time and on which side of the head they used their phones. This is called recall bias.

Mobile phones are not the only cause for development and detection of brain tumors. There are lots of factors: age, profession, genetics – all of them affecting the development of tumors. It’s too hard to match everyone, but it’s a lot easier to study multiple effects at the same time.

We’d see, for example, that healthy younger people at lower risk of brain cancer tend to use mobile phones more, and that older people sick with cancer that might spread to the brain don’t need mobile phones. Similar could hold for alcohol consumption (social drinkers tend to be healthy and social, but drinking is an effect, not a cause) and other potential risk factors.

Here’s a plot of the relative risk based on cumulative phone usage:

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It seems that the top 10% of users has much higher risk. If the data wasn’t discretized into just 10 categories, there could be interesting information here, beyond the obvious one that you need to be old and wealthy enough to accumulate 1600 hours of mobile phone usage.

[Changed the title from “many effects” to “many causes” – thanks to a comment by Cyrus]