Calculating the probability of something happening – good or bad – is a task we all do hundreds of times a day. Is the road clear enough to cross? If I have another cup of tea, will I miss my train? Do I need an umbrella today, or sunglasses (or both)?
Mostly, we do it without a second thought. We take data – how long it takes to walk to the train station, the weather forecast – and make our decisions. But for some big decisions, the data is simply missing.
Two stories I wrote this week illustrate this. The first was about IVF. Success rates are something in the region of 1 in 5 for the first round of treatment. It’s an expensive and demanding business, both emotionally and physically. So if you are among the 4 in 5 couples who don’t have a baby first time, what are your chances of succeeding in a second round?
Until recently, you’d have little firm data to go on. Estimates are based on the woman’s age, and are not especially accurate or specific. You might be told you have about a 1 in 3 chance. Or it might be less than that. How much less? We don’t know.
Scientists took data from over a thousand IVF cycles, and looked to see which factors were most strongly linked to success next time around. Interestingly, women’s age was relatively unimportant. They developed a model that could give a much more accurate and specific estimate of the chances of having a baby in the next cycle of treatment. It was 80 percent for some people; close to zero, for others. This is important, relevant information that can help couples make hugely important decisions.
Another area where hard and fast information is lacking, is air travel for people with heart disease. There have been a number of guidelines, but they tend to contradict each other, causing confusion and anxiety. For the latest set, a group of expert doctors from the British Cardiovasular Society considered all the published evidence on the topic, before coming up with advice.
Trouble is, there’s precious little published evidence. So most of their advice was based on ‘professional judgement’ – in other words, their best guess as to what specific conditions can do to people with specific heart problems, based on their experience of treating patients.
Does that mean the advice isn’t worth having? Certainly not; it’s the best advice we have based on the information available. I’d certainly consult it if I was at risk, and I’ll take their advice about avoiding deep vein thrombosis next time I fly, too. But it would be good to see some proper research in this area, too, to help us make decisions that are, for the most part, routine, but can be life-and-death on occasion.
Image: from Capt Kodak’s photostream, on Flickr with CCL.
UPDATE: The Guardian’s contract for BMJ news stories has now ended, so some of the links above no longer work. Apologies.