The 10% quoted in the British survey is not credible because of how the survey was conducted, but reliable numbers might be as high as 4%
The original survey that prompted the Question was conducted by UK campaigning website Mumsnet as part of their Rape Awareness Campaign. Unfortunately their good intentions are undone by bad survey design and some statistical illiteracy in how they quote their results.
The situation is analysed here and here by UK fact checkers Straight Statistics. The first problem is that the survey was not a properly conducted random sample but a self selecting voluntary survey. This is always a major source of bias but a good way to back up campaigns when you are less than choosy about the statistical rules. As Straight Statistics says:
The Mumsnet survey is part of a campaign called We Believe You. I certainly don’t disbelieve Mumsnet’s respondents, as it’s unlikely they are making it up. But I don’t believe that any self-selected survey of this sort provides a basis for reaching any conclusion at all.
But they also showed some illiteracy when adding up their numbers. In the second comment in Straight Statistics the problems is explained like this:
I overlooked the very strange way in which the survey was reported by Mumsnet. The PDF of the survey results on the Mumsnet website shows one question that read as follows:
Mumsnet reported this as 10 per cent reporting having been raped and 35 per cent sexually assaulted. But this ignores the third answer, those who have suffered both rape and sexual assault. Taking this into account, the true figure should be that 27 per cent have been raped and 52 per cent sexually assaulted, with only 38 per cent experiencing neither. Even worse than Mumsnet claimed!
The figures make it even more unlikely that the responses represent a true representation of women’s experiences, but the survey was widely reported, using the headline figures in the Mumsnet press release.
So they underreported the answer but didn't give a convincing account of their statistical skills.
To put this in perspective Straight Statistics quote the results of a more reliable source:
The best data we have, from the British Crime Survey (BCS), suggests this is an overestimate.
The latest bulletin on Homicides, Firearm Offences and Intimate Violence, published in January, indicates (Table 3.01) that 3.7 per cent of women report having been raped since the age of 16 (4.5 per cent if attempted rape is included), and 18.6 per cent report having experienced any form of sexual assault – around half the proportions reported by Mumsnet.
The BCS figures come from a properly-chosen sample of more than 6,000 women. The Mumsnet results come from “write-in” responses on its website from 1,609 women.
To me the much more reliable BCS results are still disturbingly high and the mumsnet survey just muddied the waters by exaggerating by bad survey design. Newspapers mostly swallowed the numbers with little critique or reference to previous, better, work. On topics as important as this they should have done a better job.
