Since we uploaded this web site we have received many questions and are, unfortunately, unable to answer all of them. We have also become aware of some common misconceptions about our work and would like to address them in this document. We hope that people will find the clarifications below useful.

Being at home versus being in the sampling zone that contains your home

Some people who have commented on our work hold the erroneous view that we assume that people are killed at home or that they must be killed at home for our model to make sense. In the model there is a zone of households that can be reached by the sampling methodology, the "surveyable" zone, and a zone of households that cannot be reached by the survey, the "unsurveyable" zone. People can be killed anywhere. Below we consider pertinent cases. Bear in mind that both zones consist of many irregular pieces all over town.

1. Consider the movements of a working-age male who lives in a household in the unsurveyable zone.

He may spend time at a local cafe, shops or a mosque that might all be rather close to his home and still be within the unsurveyable zone. Or maybe he must cross through the surveyable zone to get to his favorite local cafe, shop or mosque, but these might still be within the unsurveyable zone. He may patrol his neighborhood with a militia or vigilante group, again moving mainly within the unsurveyable zone. He might slip outside the house and spend some time on the street having a smoke or a barbeque with some friends, again not exiting the unsurveyable zone. He may travel several miles across town to work but his work may still be located in another part of the unsurveyable zone. So even someone with a full-time job may rarely exit the unsurveyable zone. He will also spend some time at home with his family, inside the unsurveyable zone. The misconception is that this time at home is the only time when he is in the unsurveyable zone.

Of course, a person may live in the unsurveyable zone and work full-time in the surveyable zone. He may also spend some time shopping and praying in the surveyable zone. We certainly do not take the position that people living in the unsurveyable zone spend all their time in the unsurveyable zone. But the unemployment rate in Iraq is high and, as noted above, employment can be within the unsurveyable zone. So it is rather generous to assume as we did for the calculation in the manuscript that the average working-age male living in the unsurveyable zone is spending 1/4 or his time in the surveyable zone. Our idea was to keep the calculation simple and that this overestimation of time spent outside zone would roughly balance underestimation for other types of people.

2. The movements of working-age males living in a household in the surveyable zone.

Similar factors to those affecting working-age males in the unsurveyable zone will be in play here so let's just consider some differences. The unsurveyable zone is much larger than the surveyable zone so people living in the surveyable zone may spend a higher fraction of their time out of zone compared to people in the unsurveyable zone. That is, to the extent that people simply wander around randomly they will tend to spend more time in the unsurveyable zone than in the surveyable zone. On the other hand, the surveyable zone is more at the center of things than is the unsurveyable zone and will tend to draw people from both zones. So the fraction of time spent out of zone may differ for working-age males depending which zone they live in, but it is not entirely clear which group will circulate more to the other zone.

3. The movements of school-age children

This is another manifestation of the same erroneous claim that we assume people to be killed at home. Children can leave their homes, while leaving their zones only momentarily or not at all. Schools are likely to be in close proximity to their homes. And when children are out of school they are likely to be close to home and, therefore, within the zones in which they live. Of course, children will spend some time out of their residential zone but this should be a pretty small fraction of all their time.

4. Women and the elderly

Our sense is that many readers already accept the idea of very low circulation in these cases. Recognizing that "in zone" does not imply "at home" only strengthens this idea. Of course, a significant number of women do have jobs and will, therefore, circulate between zones, i.e., the circulation of women is not literally zero. Again, this underestimate was meant to balance, more or less, against the overestimate for working-age males.

Bearing in mind that a person who is out of his/her zone is, of course, away from home (but not vice versa) we can pretty much rule out f values that are below 1/2; it would seem highly unlikely in any society that the average household member would spend more than half of his/her time away from home. Given the danger of being out and about in Iraq it is likely that circulation levels are particularly low.

Applying averages to individuals

It is important to bear in mind that the f values are averages over many individuals. Some people will spend more and others will spend less than the average fraction of time out of zone. We have also averaged over two populations: the population inside the surveyable zone and the population outside the surveyable zone. In the model this assumption corresponds to setting f_i = f₀ = f. Although this is clearly a simplification, a lack of information about the detailed implementation of the recent Iraq study prevents a more precise estimate to be made at this stage.

Different parameter values

Our manuscript is, by design, extremely brief. We only calculated R for one set of parameter values. However, the formula for calculating the bias factor, R, is simple enough for anyone to calculate R values for different parameter values. We have also posted the related Matlab code on our web site so that people with access to Matlab can explore different parameter combinations. For additional convenience we also provide tables and graphs here that illustrate how R depends on the parameters.

Naturally, no one can know the true values of the parameters unless the Lancet researchers agree to share information about the survey with interested academics such as ourselves. The data might also clear up a number of other questions that have been raised about the study. Thus, in our view, there is a compelling public interest in the release of these data.