Wednesday, 29 April 2009

If Red is your favourite colour are you more likely to have endometriosis?

Firstly I’d just like to point out that the answer to the above question is, quite obviously NO. There has never been any evidence suggesting that colour preference is related to endometriosis susceptibility because the two factors are unrelated. But what if there was a scientific study that said women who have red as their favourite colour are more likely to have endometriosis. Would you change your favourite colour to blue? Or green? I don’t think so, because you know colour preference makes no difference. In a similar sort of vein a study from New Zealand found that people who drive silver cars are 50% less likely to have an accident than those who drive white cars, and people who drive green cars are almost twice as likely to have a serious accident as those who drive white cars. In fact if you’re interested you can read the paper yourself by following the link here. So what should we do? If you own a green car should you rush out to the nearest shop and stock up on silver spray paint? Again the rational answer is, of course, no. The likelihood of having an accident in a car depends on the skills and factors affecting the driver not the colour of the car. The fact that silver car drivers are less likely to have a serious accident is purely coincidental, even if the study that found these associations was well designed and executed.

Okay by now you’re probably wondering where I’m going with this but bare with me. The reason I’ve put in all the preceding text is that a study has just been published in the Scandinavian Journal of Work, Environment and Health which found that women who work as a flight attendant, service station attendant or nurse may be more likely to have endometriosis. Again this is a fairly well designed an executed study, but couldn’t it all just be a coincidence like the car colours?

Well for starters, although this study seems well designed there are definite problems with it, mainly what is known as sample size. Basically if you are going to compare one group of people with another for whatever reason, the more people you have the better as it narrows the likelihood of errors occurring in the proceeding analysis. This study took 341 women with endometriosis and 742 women without endometriosis, which may seem like a lot and would be fine, but these large groups were then divided by job type. So for example, in the service station attendant category they were only comparing 4 women with endo to 2 women without, and in flight attendant category 5 women with endo were compared with only 1 without.

That’s the first problem, the second is the significance of the results. Say you had compared one group of people with another and found a difference between them. How do you know that difference is significant? Well scientists have long boring equations that you put your results into and it pops out with a number called the P value. If the P value is less than 0.05 then the result is significant, if the P value is less than 0.01 then it is really significant. Anything above 0.05 is considered pretty unconvincing. The authors of this study have put their results into these tedious but necessary equations and found that although there was a difference in the number of women with endo in these different job groups they were not significant. The nurse job group, for example, had a P value of 0.23, which is suggestive, but definitely not what is considered significant.

There is also the assumption that job has any bearing on endometriosis risk. Certain authors have said that exposure to certain environmental toxicants may have an effect on developing endometriosis and the level of these toxicants varies with job type. This has never been proven thought, so we must treat this assumption with some caution. For example there was a study that suggested women who work in wood and paper mills may be more likely to develop endometriosis due to exposure to certain chemicals involved in the wood processing industry. However, it must be remembered that no environmental chemical has ever been unequivocally proven to increase the incidence of endometriosis in humans.

In conclusion the study of relationship between job title and endometriosis is an area worthy of investigation but I personally doubt any job type confers an increased risk of developing endometriosis. Endometriosis is such a complicated disease it is far more likely that a whole host of factors play a role in risk of developing the disease. So don’t go changing your job (or your car) just yet.

If you would like to read the full article from the Scandinavian Journal of Work, Environment and Health, follow the download link on this page

1 comment:

  1. Dear Matt,

    I have found your blog via the link that you posted to my support group, Goddesses of Endometriosis. I have two university degrees, one in psychology and one in social work, and as such have taken quite a few university stats courses in my day. Thank you for taking what seems to be a difficult to understand area of research and attempting to explain it in simplified english. I'm not sure that I could have explained P values in any way that made sense to anyone! I simply plug in the numbers to the formula and go.

    I do not like the fact that there tends to be a lot of research that will attempt to link things which are not correlated, but publish them to the general society at large anyway. It is not helpful to us who live with illness, and it does nothing to increase our knowledge. In fact, I do believe that this is how rumors get started and people get stuck with the wrong information on the specific disease that they are looking at. I'm a huge believer in educating the public with accurate information, especially when it comes to reporting on research studies of value to the endometriosis population.

    Thanks again for explaining this. I figured that since I'm up at 6am, I might as well start at the beginning of your blog and work backwards up to the current.

    Take care,